You don't really need STL, just set gamescope as the launch parameter. I suggest checking --help to see what the flags do, but in general you'll want gamescope -W <res width> -H <res height> -r <refresh rate> -f --force-grab-cursor --hdr-enabled -e -- %command%. This works for me on TEKKEN 8, Helldivers 2 and Deep Rock Galactic.
There are some issues, however. Some games might just freeze when running with gamescope (or gamescope with HDR flags), which is the case for me with Deep Rock, it'll freeze merely 20 seconds after it starts. Second issue is that, at least for me, the image will be incredibly dark and for some reason the SDR content brightness slider on Plasma settings will change the brightness of the HDR gamescope window, so I have to set it to 1000 to "fix" the brightness, but my desktop will be blown out with brightness.