The wandering swordsman from FFV known for his Genji Equipment and his constant search of the sword called, Excalibur (which he end up getting an Excalipur), Gilgamesh is confirmed to be part of Chaos' side in Dissidia 012 [duodecim]: Final Fantasy and also, FFXI's Prishe will part of the roster!
His appearance was confirmed on the the trailer first seen on the first production press conference in which he popped out at the end of the trailer saying, "It's been a long time, Bartz." To those who don't know, in FFV (which I just observed my younger brother playing with), he had a heated rivalry with Bartz Klauser. The picture that is found on top of this page is his official artwork in which hackers discovered while hacking Dissidia 012 Prologus.
Aside from that, Prishe from FFXI is part of the roster as seen in the trailer. It is not sure which side she will be part off.
Sources: Final Fantasy Wikia (Gilgamesh pic, Prishe pic), Siliconera, Andriasang