I'm afraid not. The motherboard is what controls what can go in it and what not. Ones made by manufacturers, like Gateway choose to be sometimes hardware specific, and tend to use cheaper parts that won't be universily compatable.
Sucks I know, but thats the thing about some pre-built models
Your basically looking for:
Maximum Memory 2GB (Thats the max you can go up to)
PC800-40 ECC RDRAM