F-Droid is both a client and a repository. The official F-Droid repository has policies that were clarified in other replies, but it's also important to note that you can add arbitrary repositories and manage software from them with the F-Droid app.

The "build your own if you don't like it" answer is often absurdly impractical, but not here. Someone who feels it's important can put up an alternate repo containing clients preconfigured to connect to Gab and even an alternate build of F-Droid preconfigured to use it in an afternoon.