Disregard the choice of Gaia's weapon as meaning anything deeper: Nomura is reported to have given her a hammer simply because it looked cool without realizing there is no actual hammer-using class in the game. So it's not a hint at a new upcoming class or anything, but just Nomura's usual flair (as is her black attire: I see Nomura is indulging his usual tastes in character design again ).

Of course nothing is stopping Yoshi adding a new hammer class in the future but I don't think it would be likely (also GEO in FFXIV's lore are just CNJs with a different name, so they'd use wooden staves/wands not metal hammers).