The "entire premise of the fediverse," to the degree that such a thing exists, is the right of instance owners to choose with whom they federate and not. Blacklisting Gab is the fediverse working as intended.

However, Gab and the like are still there, and you're free to run your own instance and federate with them if you want. Maybe not Gab, last I heard they forked Mastodon and ripped out their federation code.