I don't think I saw the photo on the left before. It's not photoshoped, but it looks... odd. The one on the right is a commonly used promo photo. The two photos seem to be taken in the same photoshoot.
And I think I identified the exact dress Gwen is wearing in this on-set photo taken by
ronsoftie .
So I guess we can say it is Morgana who lent her dress to Gwen. Maybe she wants her maid to have fun in a ball or something. But the ultimate purpose, I think it's the TPTB's way to glamorize Gwen and develop Arthur/Gwen. In this stage, Gwen dressing like this could be for Lancelot as well. What I really don't want to see in this show is: Both Arthur and Lancelot are attracted to Gwen at the same time.