Then you don't live in the US? Europe, indo-asia etc all use 240v standard, not 120v standard. But thats only input voltage to the psu from the wall. Output voltage from psu to pc is always 12v,5v,3.3v etc no matter that the input voltage is.
So what's important for knowing just how big a gpu you can use is the output power, which should be on the side or top of the psu, stated in Watts usually, but sometimes just Amps. The 12v wattage is the most important for the gpu, without it, there's no way of telling exactly if your gpu will work or not. If the psu has a model name and number, that makes it easy to look up, but there should be an output rating table somewhere on it.