I don't look at "recommended" psu figures as they often prove to be the stumbling block for many thinking that their units meet the needs regardless of the performance and quality of the actual unit. If Gateway used the same brand of supplies as Dell and HP chances are that it is a quality unit even if it were inefficient it would still be able to meet the needs of the system.
Take off the side panel and look at the amp ratings of the 12v rails on the unit (note that some units have only one 12v rail) and post the rest of the system specs.
At idle the GTX460 uses very little power and idles lower than previous generation gpus. However load figures are the problem.
Well I am sad to say then that you are stuck with what you got unless you can prove or disprove what they had stated online by checking for your self (not there isn't software to tell you what psu you got <_<
The 5750 and 5770 don't use that much power at all so they do run on a quality 300w unit provided that the rest of the system is of low demand.
The recommend power supply numbers from the card manufacturer is about the last thing you should look at when trying to figure these things out. ATI's numbers are usually massively inflated. Nvidia's are usually semi-appropriate but you should always look at your power supply's actual power output on the +12v rail and the card's actual power usage which you can look up in any decent review of the card.
As for the HD5750 vs GTX 460 the 460 uses much more power; over twice as much on average. The HD5750 is likely the best card you can reasonable use on a 300w Gateway PSU. For anything better you will need to buy a replacement.