The easiest way aside from pulling out one of your sticks of memory would be to download this little utility, it will tell you what type of memory your computer is currently using. Here's the link... http://www.passmark.com/products/rammon.htm
I can't get my old computer to turn on for some reason, I need to do some work on it, but I think it said PC1 on one of the sticks, will your stick work
Unfortunately without knowing your motherboard make and model or a positive ID on your current memory configuration, I couldn't tell you for sure. Hate for you to get this and then your motherboard wouldn't support it. What brand of computer is it and what model? Sometimes that is enough info as well.
Unfortunately the Gateway support site is looking for a specific model number like an 812GM, for example. and the serial number service on their site says it's currently unavailable.