I just tried that feature. I added a requirement. I could only add a title and description, which wasn't great. The requirement appeared in the Issues list, which was a bit odd, and when I closed the issue the requirement disappeared from the requirements list.
Whatever that feature is meant to be, it definitely isn't requirements management. Requirements don't stop being requirements after you've written the code.
GitLab employee here - we list Requirements Management as at "minimal" maturity. I'm sure the team would love to hear more about why the feature didn't work for you - you can learn more about the direction and the team here: https://about.gitlab.com/direction/plan/certify/#requirement...