You can find tutorials e.g. here: https://www.atlassian.com/git/tutorials.
The general documentation can be found at https://docs.gitlab.com/ee/
This Knowledge Base is hosted not on the public GitLab instance gitlab.com but on an instance for german researchers: https://git.rwth-aachen.de. You can find the help at https://git.rwth-aachen.de/help.
Within NFDI4Ing we also offer workshops for git/GitLab. You can access the materials at https://git.rwth-aachen.de/nfdi4ing/education/gitlab-workshop. You can find a past event here. Upcoming events are going to be announced at https://www.fdm.nrw/index.php/fdm-nrw/versionierung-gitlab/.