Hi Jehan.
The URL with the misc directory between pub and grafik is a symlink, which has been kept to not break older links, when the the directory structure of our mirror server was reorganized.
So please use now https://ftp.gwdg.de/pub/grafik/gimp/ as the official mirror link.
Sorry, I have forgotten "Göttingen", so please use "Gesellschaft für wissenschaftliche Datenverarbeitung mbH Göttingen (GWDG)" on the mirror list.
Regarding the GPG key, I will answer later.
Best regards Sebastian