"Registration with @gmail.com addresses is blocked due to abuse, please register with another address. You can change it later, once signed up and verified."
However, apparently, you can register using a Google, Bitbucket or GitLab.com sign-in.
When someone on proggit writes — "Yeah it's really hard to submit code. Not only do you have to sign up for their site but they don't accept Gmail addresses." — they may just be repeating something they read on proggit.
