Hi.
Well, please excuse me if i'm getting your question wrong.
https://gluu.org/docs/ce/authn-guide/passport/#how-user-onboarding-works
"If there are no matches, an entry is added using the values received from the external provider"
I tried to log in using passport/github with a new github account and it works. On callback Gluu-Server creates a new user, using the username from github and voila... successfully logged in.
Martin