Gateway actually provide instructions on doing this for your laptop model. You can find it in this pdf, on Page 9 "Replacing the Memory Module"..
It looks as though your laptop will take 2GB in total, but it seems as though it may be limited to 1GB per slot on the motherboard. So basically, you would need to buy a 1GB as a 2GB stick may not work. This one looks a decent buy. You don't have to order it online, your local PC store should stock this type of memory. You are basically looking for a 1GB DDR2 533Mhz RAM Module. You can get them slightly cheaper, but Corsair are a decent brand of memory.
If you need any other info, just let me know.