Name Brand, thats about it, both company's have been around for a LOooooong time, though Gainward was bought up by some other company I forget the name, but surprised PNY isn't as expensive as many of their competitors, they used to be one of the top ranked brands of the Geforce 3/4 era... then after a while disappeared from the scene & only been back strong in the last 3-4 years...
I can vouch for the PNY, I have a 320MB GTS (my current GTS is also a 512MB, ECS brand), also have one of their Old Geforce 3 Ti-500's that still runs.
Had a Gainward GF4 Ti 4200 Factory Overclocked card (Golden Sample "Ti-4800" Equivalent card) for a while that worked well until a co-worker fried it in a system I built them around the card.
but its mainly the brand name that your paying for if the specs of both cards are the same. Gainward has almost always been one of the "performance" brands like BFG, eVGA & XFX, where as PNY was mainly the normal brands like Gigabyte, MSI, Leadtek, etc etc.
if the gainward isn't factory OC'd & neither is the PNY its most likely the name your paying for, Brand name + popularity/high demand item = higher price usually.