If my parents find out i spilled gasoline in the garage what should I say?

I think you should give a crap what's the right thing. Gasoline is flammable so you can't wait for them to ask YOU. YOU need to tell them YOURSELF. You sound like a coward. Just admit you did it. It could be very dangerous if no one finds out and there's something goinmg on in there that will cause the gasoline to ignite.