I think I may have just solved it…
I saved the list of installed packages (nix-env -q), switched to a pretty old environment generation (from like 1 month ago; nix-env --list-generations & nix-env --switch-generation $gen), ran nix-channel --update and nix-env -u, uninstalled the packages I no longer needed, and installed the packages I saved at the start. Both gimp and kiwix installed with no errors.
So I think this is it for now.
Thanks @jtojnar for your replies 