Antifa are text-book fascists. Their organized criminal behavior should be dealt with harshly - including not only the violent perpetrators but also the organizers. They are a criminal conspiracy in action.
As to junk on Gab, that is to be expected but should be no problem if users are given good tools to deal with it. User controlled blocks and filters for sure, but also means for the community to work together.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko