Because I am lazy I set up Gitlab by way of deploying a VM with a cloud hosting provider. By default it uses its public IP as the URL and that won’t do so here is how I changed it.
https://forum.gitlab.com/t/renaming-the-gitlab-host-url/685
Assuming you installed gitlab through your OS’ repository like I did,
change ‘external_url’ in /etc/gitlab/gitlab.rb
and issue a:
gitlab-ctl reconfigure