extra memory?

Yes if the memory chip is compatible.... same speed/type/pins etc

What type of memory does your Athlon PC use?
What type of memory does your Gateway use?

the answers to this question can be found by installing and using Everest ( free ) or by going to Crucial and using the memory scanner.

If the memory is compatible say both types are DDR PCxxxxx then good, but dont just rush in as next up it may not be worth adding the memory from an older machine if the Ram Speeds are not equal or very close eg. DDR PC2700 is a tad slower than DDR PC3500 and if you add DDR PC2700 to the PC3500 memory then the PC3500 speed will be reduced to the slower PC2700 speed and may or may not give you a benefit.