Banon was the guy who led the Returners before Kefka is assumed to have offed him before his time. Cid does not disappear from the island after Celes leaves, either, so Gogo certainly can't be him.

as far as I'm concerned, the Banon, Cid, Leo, Gestahl, Daryl, and whatever rumors are stretching things far too far. Gogo is just some random guy (or woman). Nothing more to it than that. If Square had wanted there to be an OMG GOGO = DARYL rumor, they'd have made it blindly obvious, and since Daryl has a grave in which she is buried, the odds of her being Gogo are pretty much nil.

(SPOILER)The Adlai quotes linking him to Gogo were fabricated, by the way, but it's become something of a tradition to mention the theory every time a Gogo thread comes up