If it doesnt work it's probably not compatible. You Gateway takes 100MHz SDRAM. Try using just the new stick my itself and see if it works. If the new stick does not work at all it could be non-supported by your bios by latency or by being double sided, or possibly you bought PC133 SDRAM and your PC 'has' to have PC100 SDRAM only. Some older units are that way. This memory is compatible though.
Further, adding to what's been said, some older systems, (in fact, even some modern systems), require memory to be 'paired up', meaning you cannot put in just one module, rather you need two, and they must be the same size.
I cannot tell if this is the case with your machine, but figured I should pass it on for you.