Hi,
Gilgamesh's leader ability is no less historical than Teddy's. He was quite happy getting involved everywhere, from the Philippines to Morocco to Japan. The "Rough Riders" as a special unit is also a bit strange. "Founding Fathers" sounds very American, but the ability attached to it seems utterly WTH: America has never changed its government so has no legacy bonus from previous governments. (Maybe this should have been given to, say France, renamed "Seventh Republic!"

)
In a similar way, the real Brazil is not known for lots of great people or for dominating the seas with battleships. (In a sense, the great tragedy of Brazil under Pedro2 is that there weren't enough *other* great people!)
As a package, Sumeria works nicely as "we are the first," straddling the prehistoric age of myth and the age of civilization. Fight off the barbarians who have yet to develop a real city? Check. Have abilities that promote diplomacy, alliances with other cities and civs? Check. UU to start things early, great for both scouting and rushing? Check. UI to start science/culture early? Check. Some of these remain useful throughout the game, as does the early overall advantage.
How the pieces actually work together has to await actual play. These are not passive abilities but must be leveraged carefully. They are also flexible. Lots of talk about early science and culture, about barbarians, about how good pillage-share is. But Sumeria is also great at simple early rushing, better even than the Aztecs.
Maybe OP, maybe UP. Hard to say.
It probably only matters in MP, however, since the AI is likely to have issues.
Anyway,
Ken