After finishing cardio at the gym and moving to weights, the Gooch Stain is what remains on the seats of gym equipment after you complete your sets. The gooch is where the sweat from your back and front collect during rigorous exercise. The Gooch Stain can be likened to the Snail Trail, albeit in a non-sexual way.
Jeff: "Alright, dude, I'm done with my sets. Your turn."
Royce: (disgusted) "Um, yeah ... would ya mind wiping off the seat there, fatty? Your gooch stain is making me want to ralph.