Some decent creatine will give you mass and strength gains. However, you need to take into consideration that creatine will cause your body to retain water (which accounts for a majority of the actual mass put on.)
I suggest going to a site like
http://1fast400.com instead of blowing your money on some overpriced and no good GNC supplement. Swole V2 and Sans V12 both gave me great results.
Good luck!