Put it all in a deep tub of water, gasoline and oil will not mix and instead float to the surface. Run the power heads deep on the bottom as to not disturb the surface to push anything out. When it seems no more gas is bubbling up, carefully pump water in and overflow the tub. The gas will pour off rather quickly.
I knocked a tiki torch into one of my ponds once and this was the method I used with complete success. A friend of mine also had the head taken off his diesel engine and a huge rain storm came through, he did the exact opposite (added oil and drained the water) and the engine runs perfectly fine. Be glad it was gasoline and not something water soluble, then you'd have a danger of killing the fish.
But chances are, any o-rings that came into contact with the gas are finished so you may need to buy new equipment.