Create a Git user for GitLab that has no login shell and provide a common name in the GECOs field (GECOS = old Unix age printers):

$ sudo adduser --disabled-login --gecos 'GitLab user' git
Adding user `git' ...
Adding new group `git' (1001) ...
Adding new user `git' (1001) with group `git' ...
Creating home directory `/home/git' ...
Copying files from `/etc/skel' ...

The result is a user being added named git, with a group called git, an established home directory, and some template files copied to the home directory from /etc/skel.