There was no requirement to blacklist gab. Mastodon clients on f-droid are allowed let users use Gab.
What f-droid does not allow is apps preconfigured to connect to Gab, or who's primary purpose is to connect to Gab.
There was even a petition to have fdroid remove an app (fedilab) that had a blacklist to disallow Gab then removed it, claiming that removing Gab from the blacklist was specifically endorsing it, and there were some people who tried to claiming that not apps that were not blacklisting Gab when other apps did meant those apps primary purpose was to connect to Gab, but f-droid weren't having it: https://gitlab.com/fdroid/fdroiddata/-/issues/1736.
However, they did consider an app that was a straight fork tracking another with the only change being the removal of a blacklist to be disallowed.