If we want to use install_gitlab we supply a repo parameter which is in username/repo[/subdir][@ref] format. This string is split by parse_repo_spec so we have a username variable (e.g., "r-lib") and repo variable (e.g., "remotes").
If my repo is within a GitLab group, then I can simply replace the username portion of the repo parameter with the name of the group.
However, if I have a subgroup holding the repo, I'm stuck. There is no params variable for subgroups.