@Gargron I visited their help centre and found this
help.gab.com/faq/what-is-gab

"you can either have an account on Gab.com, or, if you don't like what we're doing on Gab.com or simply want to manage your own experience, you can spin up your own Gab Social server that you control, that allows you to communicate with millions of users on their own federated servers from around the world, including users on Gab."

So, Gab users can interact with the rest of the fediverse, but not vice-versa.