He gets his Helmet and becomes an actual character in EX mode. Are you asking me if he's worth playing through the story mode for, or if playing 012 over the vanilla Dissidia is worth it?
Oh I just saw your other comment. Definitely get 012 over Vanilla, as it not only includes ALL of the content from vanilla, but HOURS of extra content that'll definitely keep you busy.
Yeah, you can. When transfering data, you have the option to either transfer character progress and start the game off with a few extras, or have character levels completely reset, and only get the transfer bonus items. If you played the 012 prologue and transfer your data, you unlock the Aerith assist. For every achivement you unlock in the prologue also nets you a Chocobo Cologne.
Gabranth was nerfed a bit from vanilla to 012, it was reasonable nerfs actually but he was a fantastic character in that game. Things will probably be different if he returned to Dissidia Arcades. He may get an overhaul change (IF he comes back)