GitLab seems to provide this out of the box:
Max Binnewies

I tried, but couldn’t get it to work consistently.

Google Cloud Source Repos has an authentication helper which isn’t supplied by the standard GitLab mirroring feature. There is an option to manually generate static credentials, but those use a `.gitcookie` config instead of an SSH password public key (and also run as yourself instead of a service account with more restricted access). I was able to get this syncing once or twice, but the credentials seemed to expire.