well i don't know wether gateway uses standard atx-psus or not, so that might be a problem. If it is a normal atx psu then it would be very simple to change it.
You'd need a good quality 500-600W unit to run it with nice margins for comfort. The system will use around 300W under heavy load, so you would be maxing out the stock psu. I don't suppose you can overclock it, since oem rigs tend to have their BIOSes locked up thight (overclocking an X6 will increase the power consumption (and performance) quite a lot)
1055 is a one step up from the 1035, so yours will use a little less , (this is with idling 5850 in the system using around 20W); 218W for whole systemhttp://www.techpowerup.com/reviews/AMD/Phenom_II_X6_1055T/11.html
and if you add load numbers for a 5770, around 112W (or 85 if you prefer realistic numbers), you'll end up pretty too close to cooking the psuhttp://www.techpowerup.com/reviews/Palit/GeForce_GTS_450_Sonic/26.html