Recently purchased...so you've got a Thunderbird.
Well, its either an Athlon B or Athlon C, and that depends on your FSB, its either 200 or 266mhz. I'm going to assume Athlon B.
Um, usually when you buy something from Gateway, Dell, or any other complete system manufacturer, the first thing you select is the kind of processor. So what did you order?
If you've lost that paperwork, Gateway creates a folder on your C: drive called GWCI. In that folder there are some files describing your computer. One of them is actually a letter from Gateway Quality Control listing the components in your computer. At least, this is true of the Gateway computer I have here at work.
You can also watch the machine as it boots and quickly read the information on the screen, or you can boot into the BIOS and check there or if you're really brave, you can take the computer apart, remove the heatsink, and read the top of the processor. Somehow, I don't think that last one is a good idea for you.