Merlwyb, and the "gunner" blazon as well, have been here since 1.0. Merlwyb using guns and stuff, was a tease for 1.0's musketeer (or so I believe).
Then, 1.0 failed, and ARR's development started. I don't know exactly why they left the blazon as they ignored the gun class when 2.0 came (introducing arcanist instead), but for Merlwyb, they just couldn't remove her from the game for obvious reasons (they could have killed her at Carteneau but they didn't), so yeah she's still using a gun. The other various NPCs using a gun so far have been pirates, because iconic pirate outfit includes a gun and a sword.
Then comes an officer from the yellowjackets, using a gun. In my opinion (read it as my own conclusions) it's just an other reference to the origins of Limsa. Yeah she's using a gun, but so is Merlwyb and Merlwyb isn't teasing gun job anymore. If you remember, Ninja came 6 month after yugiri's tease. 4 years of waiting for 1.0 players isn't a tease. It's just people desperately wanting to see a tease.
Again, I could be wrong. But many hints tend to show that the gunner job won't be the musketeer people want it to be.