The Arclight Association and Assemblage of Uld, are probably the 2 most obvious choices, which are currently AD’s 2 biggest and most active Gnome RP centered guilds.
But this server’s oldest surviving Gnome guild Gnomish Rescue Squad is also an option, though they are not quite as active RP wise as they used to.
Finally, as far as I am aware, there are a couple of newer minor Gnomish guilds, who’s names escapes me right now.
Here’s the link to Arclight’s forum thread:
In any case you should join the Gnomish Discord Server of AD, on which you might be able to find some of the people behind the other Gnomish guilds who’s names escapes me, that I mentioned, too:
https://discord.gg/GhjDb4
There’s also another AD based Gnome Discord server, which you’ll find here, but it has been quite inactive for some time:
https://discord.gg/va9MSZ