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
To add a few more details: After trying several times with different IPs and different browsers, I was able to register by providing only a mobile phone number once. Since that still requires personal information, this is still a very questionable process. (not to mention it took me a day to not be asked for a cred card)