Yes.
First, to clarify your question.
In GitLab, the first branch (not necessarily named master) pushed to the repository becomes the default branch which has special meaning. It is the default branch checked out by clients on a clone, and it is the default merge request target. Also, it is usually protected.
When you say "rename master", I assume you mean rename the branch, and change the default branch in GitLab.
First rename the branch locally.
git checkout master
git branch -m newname
Then push it to the server
git push -u origin newname
Now go to the GitLab web UI. Under Settings > Repository, change the "default branch" to newname.
You'll also want to make sure that newname is marked protected to prevent accidental loss by a force-push.
Then delete master on GitLab from the branches page.
With that said, please don't do this. master is an extremely well-known convention and this change will be surprising to other contributors.