I GOT A GENERAC GP500 ENGINE STARTS BUT DOESN'T PUT OUT ELECTRICITY

Check the governor and throttle for easy movement. When the load increases, the governor should advance the throttle to keep the speed up.

Check the online support of these generators, or look at this operating video for information:

Generac 5000 watt 6250 surge watt generator running

Usually generators that have stood for a very long time loose the magnetism in the ignitor coil magnet. With care, if you can find the low voltage wire into the generator 'motor' you can apply a 9v battery briefly to it and spark it into life. Use the generator if you get it generating, for a good period and don't leave it unused for a long while.
*IF YOU DONT KNOW AND UNDERSTAND ELECTRICITY DO NOT TRY THIS. GENERATORS CAN KILL*

Assuming the generator circuit breaker is on, depending on the model it could be the voltage regulator or the slip ring brushes or both. If it's a portable it could be a capacitor problem.

smell the oil to see if it smells like fuel f so the carb needle valve may not be seating

The problem is related to the engine. When a load is put on the generator, the engine's throttle will want to open to maintain RPMs. If your carburetor is misadjusted or gummed/varnished, when the throttle opens, you will get a fuel starvation and the engine will die. If the generator is used regularly and hasn't sat for a while, first check to make sure the spark plug is good since you don't want to make a carb. adjustment using a worn plug. After you adjust the carb. manuallly flick the throttle to ensure that the engine doesn't 'stutter' or die out. If it does, increase the gas in the air/gas mixture and if that doesn't solve it, remove, disassemble and clean the carburetor.

there is some problem in your AMF/ATS pannel

DO U HAV POWER FROM GEN ?

well I have a special flasher unit but I believe you can put 12 volts to the brushes while it's running. That should do it.