You can use Artifactory's Docker Remote Repository capability to proxy a Google Container Registry. The process is relatively easy though not entirely straight forward without knowing the expected inputs.
Affected Versions and requirements
1. Artifactory 4.4.3+ (preferably 5.x to support new Docker features)
2. A JSON Key File for a service account with READ access to the registry (If you do not already have one, follow Google's Advanced Authentication document.)
1. Create a new Docker remote registry repository
2. Uncheck the Enable Token Authentication flag
3. Set the URL as https://gcr.io
4. Under the advanced tab, set the username as _json_key
5. Under the advanced tab, set the password to the contents of the JSON Key File
NOTE: If you wish to migrate instead of proxying your GCR, you can use the Docker2Artifactory tool.