GNU developer here (MediaGoblin lead developer). GNU projects get to decide whether or not to do copyright assignment to the FSF. As an example, MediaGoblin and Guix don't require copyright assignment, while Guile does.
I have interacted with Tox core devs, the exchange was not pleasant. The overlap between professional and mature (both in social and engineering aspects) seems to very slim in the security space.
Remember GNU requires transferring rights to the FSF, which many aren't comfortable with.
I personally favor tox, because ec25519 and full forward secrecy. Also, not fond of SIP.