They forked off. And they did a great job imo.

Their fork of mastodon is no longer interoperable with the larger fediverse, but it never really was because all the major instances blocked gab’s as soon as they switched to mastodon in 2019 after mastodon called them bad names: https://blog.joinmastodon.org/2019/07/statement-on-gabs-fork-of-mas...