The gnome ISO on the releases page gives “503 Response object too large.” Here’s the link to the iso itself: https://releases.nixos.org/nixos/unstable/nixos-22.05pre360796.73ad5f9e147/nixos-gnome-22.05pre360796.73ad5f9e147-x86_64-linux.iso
However, it is available directly from hydra: Hydra - Build 169228082 of job nixos:trunk-combined:nixos.iso_gnome.x86_64-linux