It's not on fdroid because fdroid compiles the apk again, which poses a security risk/vector for injecting code. Gab ain't on there because they are commies. Signal is open source. Anybody can inspect it and loads of devs have. It is probably the best option for secure comms for civilians. There is a way to update it from app directly. Also check session. Finally matrix/element. There are no absolutely secure comms on internet, especially if ClownsInAction or NSA are your nemesis. Keep it clean. Feed the roots, not the goons. It's like you're worrying whether you just bought the best exercise bike. Use what you got and what is available. Also know that anytime a service decides it wants to give them your data & comms, they can. The data is theirs, not yours. In U.S. at least. With Matrix you can host your own server so data is yours. That will be the future going forward. Federation. But even then the backdoors will still give goons access, but very tedious.