Works here as well, even with the regular GIMP version. What I recognised was that an auto-update in the regular GIMP plug-in version delivered an error message, but once you click on the update button again, you’ll get the update.
In terms of usability it might be worth considering an automatic update notification, similar to, for instance, Firefox. This feature would simply tell G’MIC users that the plug-in has been updated and provides bugfixes, as well as new features. It would be up to the users to start the update process.