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
For my private repos, hosted on my home server, I moved from Gitlab to Forgejo (Git, artifacts and containers images) and Woodpecker for CI builds. Woodpecker is not as powerful and feature complete as Gitlab, but for simpler needs it gets the job done.