Gab doesn’t federate, so isn’t really part of the fediverse. They’re just using the software; they’re their own thing, not part of a distributed social network. One could make an atproto island that didn’t federate, I suppose, but it would be a lot of work for no obvious reason.

(There are a few other far-right mastodon instances, but then there are pockets of the far-right on bsky, too)