Add the following command to your settings.ini in the gamedir, 
where 1=enabled & 0=disabled.
For GODMODE simply add 

instagib = 1

Unknowns:
goreOverride - Enable/Disable Gore 
HUD - Enable/Disable HUD 
smartbomb - ? 
weakThrax - ? 
audio - ? 
music - ? 
FMVmusic - ? 
showFMV - ? 
screenText - ? 
fullscreen - ? 

(c)PiZZADOX