Since holes come in different shapes and sizes, you should open the case. Get ready for some dimmensions:
1.) SFF power supplies are~125mm wide (~100mm deep).
2.) A special version of SFF is turned sideways so it's 100mm wide (~125mm deep)
3.) Gateway's version of the PS-II (aka, full size) is ~125mm wide and ~165mm deep.
4.) Standard PS-II size is ~165mm wide and ~140mm deep.
Gateway hasn't used #4 since the mid 90's as far as I know. Which fairly means your SOL when it comes to high capacity power supplies, unless you modify the case.
Most likely you have #2 or #3. If you're lucky, you have #1 which is an easy to find part.
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