If you create a branch with the name <issue-number>-issue-description and push that branch to gitlab, it will automatically be linked to that issue. For instance, if you have an issue with id 654 and you create a branch with name 654-some-feature and push it to gitlab, it will be linked to issue 654.
Gitlab will even ask you if you want to create a merge request and will automatically add Closes #654 to the merge request description which will close issue 654 when the merge request is accepted.
Also if you go to a given issue page on gitlab, you should see a New Branch button which will automatically create a branch with a name of the form <issue-number>-issue-description.