This is not the duty of client applications. Banning instances is the duty of the operators of individual instances, and the instance(s) on which you have your account(s) are the instance(s) whose blocking policy affects you. This is a feature.
Most sane instances on the Fediverse already ban extremist and alt-right content, and Gab will be no different in that case - just another entry on the blacklist, next to https://freespeechextremist.com/ and https://rapefeminists.network and other instances who shamelessly abuse the right to free speech.