This question has been asked before on the forums, someone suggested a tool to do part of it:
You can maybe also use the Gitlab API and see what options from that would help you out:
So you are probably going to have to script it to get as much of what you want as possible. Otherwise, you probably would be easier just migrating away from gitlab.com SaaS product and running your own server since you can then use the gitlab-backup command to backup your server and then you would have everything. That cannot be done on gitlab.com.