Your feature or fix are currently done in your fork you must create a merge request on the upstream project for spreed your changes to others contributors.
In gitlab GUI open your fork (normaly in your namespace), if you've pushed your branch into your fork repository a new button has available in the GUI create merge request, so click on!
The page loading a new merging form, you must fill it! At start you must select the king of merge request in the selectbox available (example: doc/fix/feat) and respond to asked questions.
Once you have filled out the form, you must inform commiters team by ping them, so you need to send message by starting a discussion in your merge request. For that:
@commiters need review
Now commiters group affect the review to a member of the project team (it's can be a commiter or a contributor). Reviewer check the changes and can discuss about this, or directly reject or validate changes.
For rejecting the changes reviewer must send a comment to commiters group:
@commiters changes required
Now commiters are notified about the reject and reaffected the merge request to the author for changes. In this case the author must implement review comments and commit changes and ask for a new review (cf. need review).
If the new review validate changes, reviewer must send a discussion comment for approval that:
Commiters are notified of the review success and merge the request to the upstream.