Greetings!
Depending on your GitLab version, you likely can accomplish this use Let’s Encrypt (which is built in) by simply setting your external_url setting in your gitlab.rb file to use https://your-gitlab-url. There are a lot of options you can utilize with this, so I would recommend giving our SSL Configuration page a solid read. It contains a lot of details and scenarios to help you get going.
Hope this helps!