Here are the specs for the card: http://www.asus.com/Graphics_Cards/AMD_Series/EAH6450_SILENTDI512MD3LP/#specifications

On my computer, I just installed a similar graphics (512 MB) card and the power supply is 305 watts. It is working fine.

However, these are the specs for the Gateway computer: http://www.gateway.com/systems/product/529668795.php

Per these specs, it has a 220 Watts power supply. The Asus card will not work under full load conditions. Consider a PSU around 350 watts for this application.