1) Unplug the computer from power.
2) Open the computer case.
(Wish I knew the Model Number, I might be able to give you a link from Gateway Support, on how to open this puppy)
3) Look for the CMOS battery.
The CMOS battery for that computer will be a CR2032.
A type of watch battery, that is approximately the size of a U.S. nickle.http://www.staples.com/Duracell-DL2032-3.0-Volt-Lithium-Battery/product_273151?cm_mmc=GoogleBase-_-Shopping-_-Technology%3EBatteries-_-273151-DL2032BPK&cid=CSE:GoogleBase:Technology:Batteries:273151:DL2032BPK
There are several different methods of holding the battery, into the battery holder on the motherboard.
I'm not advertising for these guys, but this video tells you about BIOS, CMOS, and shows a typical CMOS battery holder.
For this particular style of CMOS battery holder, there is a spring clip that is released by pushing a tab on the side.
The tab is RIGHT next to the battery.
The tab is pressed to the Outside.
This releases the battery where a fingernail can be used to lift it up, then remove it.
Some CMOS battery holders simply have a spring steel arm, which extends halfway across the battery itself.
The spring steel arm is gently lifted up, and the battery is slid out.
The Positive ( + ) of the battery ALWAYS goes up, when installing.
BEFORE you reach inside your computer, observe Anti-Static Precautions!
Your body carries Static electricity.
Static WILL fry out (Short Circuit) the delicate hardware components inside a computer.
You may not even see it, or feel it.
Computer unplugged from power, computer case open, TOUCH the metal frame of the open computer case.
(Bare metal surface, not painted)
This action will relieve your body of Static.
Should you leave your computer in the middle of working on it, upon your return be SURE to Touch the metal frame again.
For additional questions please post in a Comment.