Anyone who wants to, including the gab maintainers, can create an Fdroid repository containing any apps they want it to, including gab, with or without the permission of the app maintainers. I run a few Fdroid repositories myself for apps that aren't on the play store or in the main Fdroid repository. The Fdroid server tool is open source. You can host your repository on GitHub or gitlab, and update it using their ci/actions/etc. It doesn't require much technical knowledge.

    pip3 install --user fdroidserver # or install with package manager.
    fdroid init # first run only. Now edit URLs and descriptions in config.py
    cd fdroid/repo
    wget gab-latest.apk
    cd ..
    fdroid update # on first run, add --create-metadata and then edit the files in the metadata folder and run update again.
    git add repo archive
    git commit -m update

This does not warrant doing that.

If the gab devs were somewhat competent they'd create their own F-droid repo and distribute their app through that - F-droid allows this, and cannot stop this.