You're not limited to a certain brand. Any brand that meets the specs will do - just stick with the well known companies.
Here's what Gateway says about your memory config:
Installed: 4 GB (4096 MB) 667 MHz (PC2-5300) DDR2 SDRAM (two 2048 MB)
Expandable to: 8 GB (four DDR2 DIMM slots)
So you can add another 4GB in a 2x2 configuration of DDR2 ram with a speed of PC2-5300. Here's an example:
Mushkin Enhanced Silverline 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 667 (PC2 5300) Desktop Memory Model 996756 http://www.newegg.com/Product/Product.aspx?Item=N82E16820226088