So please use now https://ftp.gwdg.de/pub/grafik/gimp/ as the official mirror link.
Done on web server config (for the rotation) and in our gimp-web scripts (for verifications mostly).
so please use "Gesellschaft für wissenschaftliche Datenverarbeitung mbH Göttingen (GWDG)" on the mirror list
This is now how we will list your mirror in this page and elsewhere (the change is only applied on testing website; it will be rolled out the next time we merge it into the public website).
Cf. gimp-web@bacc8770.
Regarding the GPG key, I will answer later.
Sure.