If you`re so insistant on changing things in a brand new oem computer. Why did you buy a Gateway in the first place?
You could`ve had a computer built to your specifacations.
Plus, and this has been mentioned. If you start changing things inside your new computer and anything goes wrong. you`ll likely find you have voided the warranty.
However, to answer your question. Zalman heatsinks and fans are very good. Just Google for Zalman.