I'd say maybe you slid out from under your car, went to lean back on the fender, and then realized you were to far away and banged your head into the fender!!! Ouch!!! HJoking, and welcome to the forum
PS, I don't think it means anything. Never heard of heads stamped GS. Someone stamped it themselves. JMO.
More than likely. Or by the owner. The heads on the beater say "Scalice" on them which was the guy's name I bought the car from