Right now there's no way to force gamemode session to run at an arbitrary resolutions. Many devices would benefit from this, especially older laptops with weaker specs yet high resolutions.
I was trying to set up Bazzite-Deck on a secondary laptop (older Lenovo Yoga 2-in-1). Gnome runs fine when I disable blur-my-shell. Gamemode is too heavy at 1080p for its integrated gpu (7th gen i5 Intel Graphics).
My idea was to be able to use game mode as a way to leverage all the meagre resources of this machine for some lightweight gaming and in-home streaming, but I couldn't figure out a way to lower Steam's BPM resolution, even after tinkering with the remote CEF debugger (developer options) . I got as far as to make the resolution selector appear, but I only get 1080p60 as an option (besides Automatic)