I've used gitlab omnibus installation version but my PC had broken so couldn't boot my PC now. So I couldn't run gitlab and have to make the backup from this condition. From Gitlab documentation, there is a description how to make a backup on gitlab running state but there isn't any description way to make a backup on not-running state. (https://docs.gitlab.com/ee/raketasks/backup_restore.html)
The repository is already backuped and what I really want to make a backup is about gitlab support functions (e.g. issue, merge request and etc) How could do this?
If possible, you would need to backup the data mounted by your GitLab omnibus image, and copy that data on a working PC, in order to run GitLab there.
Once you have a running GitLab on a new workstation, you can make a backup there.