Find this file:
\My Documents\My Games\Gas Powered Games\Demigod\Game.prefs
Find the option section
options = {
...
texture_level = 0,
fidelity = 2,
primary_adapter = '1280,720,85',
music_volume = 92,
fx_volume = 91,
...
},
Set the primary adapter to what you need (hor,vert,hz). If your machine and driver supports it, then the games start with that resolution. Otherwise you'll get an error.