No one is stopping Gab from speaking.
FDroid is just refusing to amplify their speech.
Since when did "free speech" == "freedom to force other people to publish my speech"?
But you know what, here's an idea: I have a sign I want to put on your lawn. I'm afraid it's an incredibly offensive sign and you and your neighbours might not like it. But I wanna put it on your lawn anyway.
Judging by your comments here, I assume you're totally cool with that, right? I mean, according to you, denying my request to put my incredibly offensive lawn sign in front of your house would be an abridgement of my freedom of speech, right?