Currently, Gitlab has an integration through an add-on and it only works on Jira Core projects, so it's not possible to integrate with next-gen.
We have a feature request suggesting Jira support Gitlab the same as its support Bitbucket/Github:
Please, click on vote and watch to receive updates about the feature.
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I searched for updates regarding this integration and couldn’t find any information for team-managed projects.
I installed the add-on GitLab for Jira and it shows the “Add feature flag” option on team-managed tickets.
Have you tried following the steps of their documentation to integrate GitLab with Jira?
On the feature requests tickets, there is no further information about the integration: