How to prune branches in Gitlab

My project existed for a while and beginning to be populated with old branches. I want to prune these branches. I know how to prune git branches but don’t know how to update Gitlab with the new pruned branches.

Is there a tool or gitlab-rake option to update Gitlab database with the prune git branches to delete rows or do I have to create a new project and import the pruned branches? In the later case, I would loose all my merge request information.

Any help will be appreciated.

prune remote repository is useless, since GitLab run a periodical job to do that.
Deleted branches are no more visible in GitLab immediately

If you still view old branch in from you client you probably need to run:

git fetch --verbose --prune