As others have stated, launching without the gamescope options will result in a mostly black fullscreen window. With the gamescope options, though, KDE will not position the window correctly.

I do not know the equivalent for other DEs/WMs, but for KDE/kwin, you can put the following in your ~/.config/kwinrulesrc:

[7f96cf86-2f88-4bd9-ab94-43e2c6914ad1]
Description=Window settings for Rusty's Retirement
above=true
aboverule=2
noborder=true
noborderrule=2
position=0,1144
positionrule=2
size=2560,296
sizerule=2
title=Rusty's Retirement
titlematch=1
wmclass=gamescope
wmclassmatch=1

Note that this config is for a 2560x1440 monitor. Edit the position and size fields appropriately.