Gun healer would be more likely a variant of the Flinklock from Tactics A2 - http://finalfantasy.wikia.com/wiki/F...8Tactics_A2%29
Agent would be more like a DPS according to it's skill set, plus support skills. These two could get mixed together to create a new version of Agent.
Let us not forget this little bit from Yoshida in an interview prior to 2.0 release
"Musketeer is planned for after that and also thinking of duel gun"
The "after that" was referring to the release of Thief and Dark Knight