If your IP (and possible your browser) looks “suspicious” or has been used by other users before, you need to add additional information for registration on gitlab.com, which includes your mobile phone number and possibly credit card information. Since it is not possible to contribute or even report issues on open source projects without doing so, I do not think any open source project should use this service until they change that.
Screenshot: https://i.ibb.co/XsfcfHf/gitlab.png
Tried to register with gitlab three times some months back to file a bug against qemu. It rejected my registration silently every time (as in, it appeared to take it but never sent a confirmation email, not even one that got mistaken for spam). I gave up on filing the bug.