To upgrade the memory of a Gateway computer, first disconnect it from the power source, disconnect all cables from the CPU, and open the CPU casing. Next, locate the expansion slots on your motherboard. If there is an unused slot, gently pull away the plastic tabs away from the sides of the slot, and insert the new memory module into the slot. If you insert correctly, you should hear a click as the tabs secure the memory module.
If all the slots are in use, gently pull away the plastic tabs away from the sides of one slot, and remove the memory module currently in place. Next, carefully insert the new memory module into the slot, and press gently until the plastic tabs secure it. You should hear a click. Close the CPU case, reconnect all the cables you detached from the CPU, and plug the power cord into the power outlet. Turn on the computer, and check whether the operating system is reflecting the new amount of RAM in your computer.
Only RAM that is compatible with your machine can work. To check what kind of RAM your computer needs, visit the official Gateway website, and search your computer's model. While you're there, also check the maximum amount of memory your computer can handle.