Install GitLab on server with existing Git setup

We’re currently in the process of seeking a GUI to add to our existing Git setup, we’ve looked at a few options but GitLab seems to be at the top of the list due to the wonderful documentation and community backing. One of the things holding us back from moving forward with the integration is the feasibility of seamlessly adding it to our stack without interruption or data loss.

After searching through the forums here, and more generalized development forum I have yet to come across someone outlining exactly how this process is handled with GitLab. Is there a way to integrate an existing Git setup directly into GitLab during the install, or does each repository have to be migrated manually after GitLab is installed?

Is it possible to install GitLab on a server which has a standard git server running without disruption of service, manually updating the refs for each repository as time goes on?

Any insight into this process, or tips on how to achieve a smooth transition into better version control is appreciated!