People's reactions to Gab joining the Fediverse has done, and will continue to do, far more damage to the communities here than Gab itself.
For example, many instances are blocking https://mstdn.io because they refuse to block Gab, separating people from their possible and actual friends on the network. This is harmful.
Defederating based off of who other instances federate with will harm adoption. When a user can't follow their friends on most nodes, they'll lose interest quickly.