We have a main repository based on Gitlab CD/CI. Now with our grows, we had a hard time to update each of our customer's apps using auto deploy with Git.
Now we're looking for a solution when production stage success and tests passed then all of our customers get the latest deployment and pull updates automatically. (Continues Delivery)
There is a deploy to production in Gitlab but it goes for single production we want to duplicate this steps for each mirror.
Gitlab-CI deploy stage can have multiple environments, which you can set each to automatic/manual deployment.
Example: just change the staging to customer1, production to customer2 and add as many as you need.
Another solution would be in application automatic updates if you dont have access to your customers environments.
Also checkout Spinnaker which seems to be designed for CD specifically.