Appearantly Gnome does not implement the idle protocol. I therefore had to update my Slideshow for Gnome. It ran perfectly on KDE with Wayland using SwayIdle, but for Gnome on Wayland I had to come up with something else. Luckily the command
dbus-send --print-reply --dest=org.gnome.Mutter.IdleMonitor /org/gnome/Mutter/IdleMonitor/Core org.gnome.Mutter.IdleMonitor.GetIdletime
gives the idle time on Gnome. With this function in place my slideshow works on KDE and on Gnome with Wayland.
https://gitlab.com/raginggoblin/slideshow