You have the following methods for connecting to a private gitlab repository
With SSH
git+ssh://git@git.mydomain.com:Username/Repository#{branch|tag}
git+ssh://git@git.mydomain.com/Username/Repository#{branch|tag}
With HTTPS
git+https://git@git.mydomain.com/Username/Repository#{branch|tag}
With HTTPS and deploy token
git+https://<token-name>:<token>@gitlab.com/Username/Repository#{branch|tag}