Okay thanks, I was able to reproduce the bug where the option to switch to GitLab sign up isn't appearing: https://mattermost.atlassian.net/browse/PLT-4713
We're going to try to fix this for v3.5 (November 16th release), after it's fixed the email users should be able to log in with email and then go to Account Settings > Security > Sign-in Method and switch to GitLab.