First off, you should probably do some research on your own regarding what rooting is and what it MEANS.
Second, I have no idea what Game Killer is and what it does. It's not in the Play Store, so where are you getting this app and what does it do?
Third, If you give an app root access, you better be SURE what it needs root access for. It can cause some serious problems to you and/or your phone.
If you still want to try rooting your phone, give this thread a read. http://forum.xda-developers.com/show....php?t=2150175
If you get bored reading it, then rooting isn't for you.
Current Smartphone: Samsung Galaxy S2 Telus 4G
Original purchase date: January 2011
Current ROM: Beanstalk 4.4008
Kernel: UBER Kernel 4.0
Jedi Knight 6