Bartłomiej Piotrowski commented:
I have reworked Flathub builders donated by Codethink as GitLab runners, they're available at aarch64 and flatpak-aarch64 tags. (I lost their original hostnames so I don't know if these were gnome1 and gnome2 unfortunately.)
Closing it as from infrastructure side it's done; the remaining item is providing gnome-runtime-images for aarch64 which I will try to look at nonetheless.