!GNUsocial uses the !OStatus protocol for federated communication. I believe !Diaspora partly implements OStatus, so a GNUsocial user can subscribe to a Diaspora users, but a Diaspora user cannot follow a GNUsocial user. I believe !Friendica does a better job than Diaspora of implementing both the OStatus and Diaspora protocols, so there may be people who use Friendica as their central social network, then federate that to GNUsocial and Diaspora, and maybe even Twitter. Join the !GNUsocial, !Diaspora, !Friendica and !OStatus groups and you'll get better information than I can supply. I don't (yet) run Friendica, but I'll set it up once I collect my Copious Free Time.