Yeah, I think that gab has done some work to prevent their users from interacting with the broader population. Spinster had a post to this effect earlier today.
It's something of a blessing for us since gab is really annoying but I just wish that if they wanted to run an isolated instance they would do so instead of pretending to federate.
Honestly, for someone their size, a normal bb with phpbb would be fine. There's not really any reason to join a fediverse if you do not want to federate.