NOTE: This is out of date until I update it according to Ask Creative Development - Round III Answers. A lot of what originally appeared solid is now contradicted. Since their lore seems sparse as hell, I figured I would go and chase up every teeny bit of lore I could find (Namely npc quotes). From taking the quotes and running with them, I think I've actually managed to discover a bit more without just making up my own stuff.
Quotes [Quotes] "That reminds me, someone was asking about you. That harvest-witch, Celestine; she said the winds spoke your name. You'd best hurry, . We're going to need you to get back to the worgen-smashing once you're done conversing with nature." Gwen Armstead (Within Gilneas)
"We have been blessed today! You are alive and well!" Celestine of the Harvest (Within Gilneas)
"We were driven to the edge of extinction once before, . But we druids, keepers of the old ways, saved our people from famine. When we cut ourselves off from the outside world and our crops failed, it was our order who called upon the earth's blessings and restored the harvest. Just as we did then, it is best to put our fate in the hands of a higher power. Join me now and learn what the wild has to teach us. " Celestine of the Harvest (Within Gilneas)
"The earth speaks to us, . It will not abandon us even in these perilous times." Celestine of the Harvest
"Not bad at all. Perhaps there is still hope for the old ways." Celestine of the Harvest (Within Gilneas)
"A beautiful city, is it not? As was Gilneas, in its time. But such things pass, as shall we. Only the earth and her secrets will remain. Are you ready to learn, ?" Celestine of the Harvest (Within Stormwind when spoken to by a druid player character)
"You are not initiated in the old ways. There is nothing I can teach you, child." Celestine of the Harvest (Within Stormwind when spoken to by a non-druid player character)
", I am not surprised to see you here. I always thought you were my brightest pupil. I am honored to fight alongside you, my friend." Celestine of the Harvest (Within Mount Hyjal)
Then the massive crows morphed. Genn was still getting used to seeing this transformation. He had heard that druidism was practiced among some of Gilneas's agrarian folk, but he hadn't been exposed to it until recently. The bird shapes twisted and jerked, wrenching their anatomies into their more natural forms -- those of Kaldorei druids, two males and one female. Lord of his Pack (Chapter four)
"Merry met, ! Hallow's End is a time for celebration and reflection, though many of us have forgotten why. Indeed some aspects of the festival, like the Wickerman, have been lost to time... until now, with the return of the Gilneans to the Alliance fold. Travel outside the gates of Stormwind and speak to Celestine of the Harvest. She can tell you more about the Wickerman ceremony." Alliance Commoner, during Hallow's End
"The Wickerman ceremony is a time honored tradition. It is good to see the Alliance celebrating it once again. " Celestine of the Harvest (Elwynn, during Hallow's End)
"Greetings, . The Wickerman you see before you symbolizes self. The flames that burn it are a cleansing fire. Tradition states that we take our burdens and give them to the fires. We let go of our burdens as we approach the winter season and we gain clarity and peace. Join us in our tradition, . Take a branch from the nearby bonfire and hurl it at the Wickerman." Celestine of the Harvest (Elwynn, during Hallow's End)
"Do not let your burdens trouble you any longer. Any time you are feeling burdened, remember the Wickerman." Celestine of the Harvest (Elwynn, during Hallow's End)
Names and titles A Gilnean druid could have a name like any other Gilnean - one that brings to mind Victorian times. One named Gilnean druid, Celestine, has a name with Latin origins so Latin names could possibly come with their culture. You can find Latin names with English equivalents here and Victorian era names here. As Celestine appears to lack a surname, this may not be uncommon among other Gilnean druids, perhaps due to years and years of seclusion from society. The title 'of the Harvest' was granted to those druids who saved Gilneas from famine. They refer to themselves as druids and keepers of the old ways, whilst others called them witches. It is possible that the term witch could be resented by your character, as it is likely a slur brought about by misconceptions of their magic and the resulting genocide. Others may embrace it, perhaps in an attempt to reclaim it and have it and their kind associated with good.
General overview They most likely lived in the countryside or forests, closer to the wilds and with other druids in their order - and away from the people who harassed them out of suspicion. Some people working on the farms were druids, which may tie into their religion, such as tending to plants and the food chain. It is unknown if they had shapeshifting capabilities prior to joining the Cenarion Circle - in the starting zone Celestine can teach some forms shortly before you find the Kaldorei, and a quote in Lord of his Pack implies it could be possible, though as there is no quest surrounding this and shapeshifting is very time-expensive and specific to learn it may be just game mechanics letting you learn it before leaving Gilneas. Their skills are a possibly weaker form of druidism involving the Balance and Restoration-related abilities, definitely something that can fix up plants. You are taught the Rejuvenation spell in a starting quest so healing people should of been within their capabilities as well. They seem to have a bit of shamanistic-esque skill too, with Celestine claiming 'the winds' spoke someone's name. They revere 'the earth' which they refer to as her, perhaps a mother nature concept of a goddess similar to the tauren and the Earthmother. They believe she communicates with them, will not abandon them and they ask for her blessings, learning from what they see in the wild. They were likely lower class or working class people, living off of necessities - if all things shall pass except for the earth and her secrets, why live in a fancy house when you can stick around with nature?
History The Gilnean form of druidism has been around for a long time, evidenced by its closeness to nature and it being referred to as 'the old ways' by its practitioners. It could perhaps be related to the tauren's beliefs in the Earthmother from before the Sundering that separated Kalimdor, but this is a bit of a stretch to believe, lacks basis and would not come up in your druid's Rp. For a long time, they kept themselves secret to avoid being harassed or even killed by those who viewed them with suspicion. Being driven to extinction by starvation instead isn't likely as they could of tended to their own crops away from others in peace. As they have initiation rites or procedures before they are able to teach an individual their way of druidism, it is likely that their rituals may of came across as something akin to a group of shadowwy cultists or warlocks instead, which may of angered priests and townspeople into slaying them out of paranoia. The Gilnean druids saved their country from famine by aiding the failing crops after either the wall's construction or the connections with the Alliance being severed, closing off any potential imports from other kingdoms. This would of happened sometime within the last twenty years, either closer to twenty years ago (After the Second war where the opening cinematic states they drew inward and began construction, though this didn't stop Northgate Rebels storming the gates after the Third War) or perhaps ten years ago at the least - when the wall was definitely fully built and the Scourge were striking it in the Third War. After this act the people no longer tried to get rid of them, perhaps even hailing them as heroes. It is this act that got them dubbed harvest-witches instead of just witches by the non-druidic folk. The title 'of the Harvest' was likely granted after this act to Celestine. When the Kaldorei came with their ships they were able to learn more about druidism, perhaps learning how to take on animal forms. In present times they still practice their ways, working with the Cenarion Circle and joining their battles against anything that would threaten the natural order - or the world.
Reception The Gilnean druids no longer hold an outlawed place in society. Some may still view them with suspicion and distate, some with respect for their deeds and guilt over hunting them for so long - but either way they saved Gilneas' lives. Even if a Gilnean still distrusts their magic and practices they may view them as necessary at the least, and nowhere near as bad as actual cultists. They are very likely revered as heroes for saving everyone in Gilneas from starving to death. Asides from that, there is a bit of a culture clash - living closer to nature and doing agricultural work could get snubbed by people like nobles, and no proud Gilnean would want to hear them go on about how Gilneas won't last forever but their deity they can't even see or comprehend as being the entire world will. Witch-hunting them would not be looked upon kindly anymore. Expect to be brought up on murder charges for it.
Rituals and Things Initiation ritual - it is unknown what this consists of, but a Gilnean druid cannot actively mentor someone in the Gilnean way of druidism until they have undergone it. Hallow's End - this time of celebration and reflection on the past is of Gilnean origin, with Celestine playing a part with the Wickerman ceremony suggesting a druidic origin. Though the fact that the witches were being hunted at the time can rule that out, there is nothing stopping them from getting involved now. A twig is thrown onto the burning wickerman to symbolise letting go of burdens. The Alliance had forgotten the holiday's origins until the Gilneans were welcomed back into the Alliance.
Filling in the blanks Though I hope the above is informative, a lot is still left blank regarding this race/class combination. This can make any Rp that centers around your class difficult as questions are left unanswered. It is acceptable among roleplayers to make up your own sub-groups and small factions to an extent, else any Rp guilds that do anything as a group would be deemed lorebreaking. It is not unreasonable for an individual to strike out and form their own gang, after all - just not, say, an entire kingdom. You could imagine what your own druid's order or sub-order in their given area were like - did they keep in contact with the rest of the Gilnean druids? Did they not? Were they stricter than most? A family group? What were their own initiation rites and rituals like? Were they working on a farm and hidden right under the public view's nose or living in secrecy in an obscure place closer to the wilds? If you do choose to get plotting about it, just bear in mind that they are druids and it is mainly the other Gilneans who were calling them witches(Though perhaps yours chooses to go by harvest-witch or witch to reclaim the title in a sense?) - cursing people, necromancy and other spells more befitting a darker class like a warlock do not make sense for druids. Possible inspirations for sub-groups/covens/guilds/whathaveyou are:
Wiccan religion - likely what the harvest-witches were based off of, at least in names, historical victimization and stereotyping. They believe in magic, balance and deities. This FAQ helps debunk stereotypes and misconceptions, though your mileage may vary on the rest of the site. This one is helpful too.
What to avoid: running around cackling in pointy hats and cursing people. You can't do curses with nature magic anyway. I wouldn't go making up your own deities either as an entire deity made up for your Rp is really taking liberties with the lore. If you like the concept of revering a deity, go for the planet Azeroth (Which harvest-witches are pretty much worshipping anyway), parts of it or leave the deity's actual existance as ambiguousity leaning towards a 'no'.
Romani - not a religion but an ethnicity with loads of subgroups spread worldwide, originating from India and areas that are now Pakistan. Some caravan models ingame validate the possibility of nomadic groups in general existing, some being distinctly Gilnean in appearance. Historically Romani have been nomadic to escape persecution that still thrives. Some stay nomadic, some don't. They have traditions and rules like anyone else, some aspects of Hinduism like the concept of karma, etc. There is an FAQ on Romani and the word gypsy here and a load of links here (Trigger warning for racism, rape and genocide as topics), including links to things like traditional dress, though apparently not all wear traditional dress.
What to avoid: the word gypsy, crystal balls, child-kidnapping, thieving, bellydancing, dog-eating...There are a whole load of misconceptions and stereotypes going about that hide away what they are actually like. If you plan on basing your group off of any nomadic people then please do some research, as the whole eats-crystal-balls-for-brekfast tropes leads people to believe nomads are actually like this or that they're just a thing of the past and fairytales.