https://gitlab.com/users/sign_in in Tor Browser leads to a 503 error and an infinite redirect.
I expect to be able to be presented with a CAPTCHA, or whatever CloudFlare require and upon solving it reach the login page and be able to login to my account.
I use many sites configured with CF DDoS protection with Tor Browser. GitLab is the only one which suffers from this infinite redirect problem. I therefore suspect it must be due to incorrect CF configuration on your side.