Ametrine raises an important point I mentioned in my previous post which warrants further mentioning as to why it's highly unlikely you will see any Garlean as a playable character in Dissidia. The development team for the previous Dissidia titles on the PSP mentioned in contemporary interviews that their design brief for the game was that each character was to be unique in both play style and appearance (which they found very difficult to do), hence this is why for instance Vivi from FFIX, despite being a highly popular character, was never added to Dissidia either, as there is already a BLM character in the game (FFXI's Shantotto).

As I mentioned earlier Gabranth from FFXII is already in Dissidia and so satisfies the 'Judge' type character in fancy armour connected to an evil empire, meaning, no Garleans are possible (lets face it, Garlean legatee like Gaius are clearly based on Archadian Judges from FFXII). And a final nail in that coffin, there are already gunblade users in Dissidia too (Squall, Lightning) so even their weapons wouldn't be original. Hence I am going to once again reiterate that I do not think it likely there will be any Garlean antagonists added as playable characters in Dissidia.