App Store is about central authority. F-Droid is about freedom to add whatever repositories you deem useful. It's up to operators of F-Droid-the-repository to decide what they want to host. But it's up to you to add another repository to the list. You say there's no other repository hosting Gab? That is not a problem that can't be dealt with. Unless network operator decide to ban routes, it's just a work (setting up a repository and trust) that needs to be done.

EDIT: This is mentality I find hilarious. It's either I can something for free or I can't get it at all. I think freedom in this case is about having something with little expended work.