Free as in free speech. The Gab app does not violate the four essential freedoms [1] or F-Droid's inclusion policy [2]. They even host Hentai apps where you can access lolicon pornography [3] and simply state that users should "be aware of any applicable local laws and regulations".
[1] https://www.gnu.org/philosophy/free-sw.html
[2] https://f-droid.org/en/docs/Inclusion_Policy/
[3] https://f-droid.org/en/packages/moe.feng.nhentai/