The model# of the gateway (and the gpu make / model) as well will help. Check out the gateway support page
and put your serial # in there and you will find a parts list. Google the PSU part # for replacements and you should get a good idea of the dimensions. They have decent pics of the case / psu as well and can get a good view of the mounting requirements.
Gateway sometimes has standard ATX but strange mounting brackets, somtimes a standard ATX will fit or not.