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
I remember when gitlab.com was the most accessible alternative to GitHub out there, but it seems they're only interested in internal enterprise usage now. Their main page was already completely unreadable to someone not versed in enterprise tech marketing lingo, and now this.
Thankfully Gitea and Forgejo have gotten better in the meantime, with Codeberg as a flagship instance of the latter.