Google Code will be shutdown on January 2016 ([[ http://google-opensource.blogspot.com.es/2015/03/farewell-to-google-code.html | blog page ]]) and during 2016 only tarballs of the projects code will be available.
Is there a plan to import repositories that haven't been migrated from Google Code to our Gerrit? Maybe doing a triage and import only the ones that may be worth keeping, like [[ https://www.mediawiki.org/wiki/Extension:Jpgraphmw | jpgraphmw ]]? Letting them die without backing them up is also an option, since extensions not migrated are probably broken and unusable in the currently supported MediaWiki versions.
We have a list of extensions in [[ https://www.mediawiki.org/wiki/Category:Extensions_in_Google_Code_version_control | Category:Extensions in Google Code version control ]], although there may be others that don't use the `{{GoogleCodeDownload}}` template.