As already mentioned there are:
The Arclight Association, The Gnomish Air Corps, Assemblage of Uld and the, I believe, oldest all Gnome guild on Argent Dawn, Gnomish Resque Squad, though the latter is not all that active with RP anymore outside their weekly guild meeting and the big annual memorial service they host in New Tinkertown.
And then there is this relatively new guild that might be worth cheking out as well, Copperwheel TDI:
https://eu.forums.blizzard.com/en/wow/t/a-n-rp-copperwheel-tdi-recruitment/
Good luck finding the right guild for your Gnome, fortunately there are more options for that currently than there have been in a long while.