Having cleared it with the mods I thought I would give you links to my ongoing crossover series which focuses on Gaius from his youth to present day. I'll post a general header for the whole series, but the individual links will take you the fics on my LJ and there are specific headers for each one there.
Series Title: Myth and Magic
Author: A Lanart
Fandom: Merlin/Highlander
Characters: Gaius, Blaise (omc), Uther, Merlin, Arthur, Leon, Gwen. Methos and Darius from Highlander
Pairing: Gaius/Uther
Rating/Spoilers: G for series. Spoilers up to S4
Warnings: none (but see spoiler alert)
Summary: There was a time when magic was a part of life and beings that became myths walked the earth. What most people didn't realise was that some of those living myths-to-be would continue to walk the earth, for they were immortal.
Disclaimer: BBC own Gaius, Leon and their reworking of the Merlin myth.
Panzer/Davis Productions own their concept of immortality.
No copyright infringement intended, no profit made.
In chronological order:
Illumination Shadowplay both of these involve a younger Gaius, before he has even met Uther.
The rest are present day fics, focusing on Gaius.
Heaven and Alchemy If Only and Sometimes (strictly speaking this is a Merlin centric fic but there are some important references to Gaius' place in the series in it)
Brothers in Arms Our Farewell (contains major spoiler for S4 ep 3 "The Wicked Day")
Please mods, a type:crossover tag would be nice.