Almost certainly the problem is old fuel, the carbs will definitely need the old fuel drained from the float bowls and likely a good cleaning of the jets. If you can coax the revs up by using the choke, which is actually a fuel enrichening circuit, that supports the theory. It is either lacking fuel or air so another place to look is the air cleaner, could be the air box is packed full of mouse nest material, I’ve experience that problem myself more than once. Buying any non-running motorcycle is risky unless it has very low milage and you are an experienced wrench. GS500 is a nice bike, so I hope it works out for you.