There is already a "gun class guild" in the game. It shares the same building with the Marauder guild in the Coral Tower of Upper Limsa Lominsa. Has been there from 1.0 iirc.

Inside you will find a gun toting guild leader and a bunch of npcs practicing with their guns in the main area and the target practice area of the basement.

Also Merlwyb Bloefhiswyn the leader of the city state wields guns as well.

E3 Interview-Machinist!
MMORPG: What’s your favorite Final Fantasy class that is not in Final Fantasy XIV.

Yoshi - P: I really like Cecil’s story from Final Fantasy IV, so the Dark Knight. From Final Fantasy Tactics, the Machinist.

From April Live Letter at the Fate Event;
YoshiP saying we’re going to do gun, but it’s not something what everyone is thinking it’s in the plans, it might be earlier than what everyone is expecting. It’s not 2.3 but it’s not too far out. We still got E3 and other events we need to keep good infos.

From Massively E3 Interview;
M:Are we still going to hear about that gun-using class that was hinted at?

Y:We talked about it, and I'm not one to lie to the fans, but it's still a little bit early to talk about it. When I can talk about it, I will.