http://www.tftcentral.co.uk/reviews/acer_predator_x34.htm Review just makes it sounds like game modes are basically the 3 user presets.
Maybe you are confusing it with the Z35? Apparently on that one you need to run it in the first game mode to use the 200hz overclock
game modes on both monitors are just saveable preset modes, nothing more. you can use 100Hz on the X34 and 200Hz on the Z35 (Assuming your system supports them properly) just staying in the standard preset mode, or any of the others if you want.