I was gonna fill our tractor with gas and there were to caps, a metal one a d a filter looking one, neither was labeled so I figured the metal cap was the gas cap, I put gas in it for just a few seconds an it exploded out, it was then drain in out a tube under the tractor. I think a lot of it came out and I left the cap off for awhile to let it evaporate hopefully, will it be fine?
Flush and refill as soon as you can as kmrash said.
I would also refill it with water right after draining it run it for a little then drain the water. After this add the new coolant.
To drain it there should be a valve at the bottom of the radiator. If you do not see one you can always remove the lower radiator hose from the, radiator not from the engine (radiator is usually the lowest point), and let it drain out that way. Again flush it with some regular water then refill with a proper mix of coolant for your climate.