Yeah, a Phenom II X2 550 or Phenom II X3 720 would be a good choice for some light gaming. AM3 CPUs do work in AM2+ motherboards, but I'm not sure if your motherboard would support it, you may need a BIOS flash, I'm not sure if updating Gateway's BIOSes will add support for AM3 CPUs, but if it does then good. Otherwise, I don't think you can.
I looked up your computer and it only shows one chip as an option, so you may only be able to use that chip with the bios on that motherboard. You would need to contact gateway to find out if the bios supports any other chips.
Ok I'll look at it once Im off of work. If it doesn't support amd3 socket. What are my options In that case, get a new mb as well?
If you get a new motherboard you will also have to get a new copy of windows because the os that was shipped with your pc will know that the motherboard has changed. Also if you upgrade to an AM3 board you will have to buy DDR3 ram because your system currently has DDR2. The biggest problem with buying a brand name computer is that it is hard to upgrade the core parts without upgrading everything.