The beauty of power supplies is they're all standard. You don't need to get a gateway brand power supply, just find another somewhere that will fit inside the case and is the same style as the gateway (at or atx style, though i'm 99.9% sure you've got an atx).
And just a note, in atx style systems, the motherboard decides whether or not the machine boots up, not the power supply or the switch. In other words, a faulty motherboard too can prevent a power supply from powering up.