It is a privacy leak to tell Github about what other websites or products you are using, that you otherwise wouldn't if you just used normal email/password signup.

GDPR only has a tangental relation to privacy. It might be "GDPR compliant" for me to publish my nudes to twitter, but I've still (voluntarily) lost some privacy for doing so.

There are tons of problems with GitHub or Google only. What if you use ProtonMail and GitLab? (or any other of 20+ email / code repo providers)

I don't own either service.