Now, I think this would be cool. There's actually not a GG for Attila. However, you could simply add a GG at the start and rename it Attila. Just go into WB and add the GG unit and then rename it when you exit out. Use the field of view to zoom most of the way in so you don't spoil the map (not quite all the way though as I had trouble adding a unit that close to the ground)
The caveat is that the GG must be added to a Cataphract unit and can get any promos other than Medic II+ (it can take medic/march). In other words, Attila will be an attack unit. A super medic can be created from a GG you generate later and I expect there will be many to come.
Adding the unit at the start is simply so that Attila is there for everyone who plays along. He can simply govern in the capitol city until Cataphracts arrive, then head out to lead the charge.
Not sure what to do about the cities naming convention.
(By the way, Attila is the generally accepted spelling, but technically the double L is correct too - it's the Turkish form of the name.)