Please see this guide about on-premise connections first before connecting to self-managed GitLab.
Create an application
You will need to create a new oauth application for Gearset. To do this, navigate to Settings > Applications within your GitLab instance and create a new application.
Gearset as the name, and
https://app.gearset.com/authorization/gitlab-self-managed/callback as the redirect URI.
Ensure that Confidential is ticked and then tick the
Click on 'Save application'.
Add to Gearset
To add your new GitLab application to Gearset, click on the + Connect to self managed GitLab button on https://app.gearset.com/linked-service-connections and fill in the on premise GitLab url in URL, and the application ID and secret as shown in GitLab
Connect, you will be redirected to GitLab where you can log in normally and accept the requested permissions.
If this is successful, you should be able to run comparisons from repositories stored in your GitLab instance