I noticed that when I'm parked downhill my car has a rough idle. It kind of feels like I'm running low on gas. Also, when I'm parked downhill the gas hand is lower than what it should. Everything turns back to normal when I'm on a leveled surface. Does anyone know whats the cause of this?

That'* just how our gas gauges are. If you're low enough on gas when parked downhill, it could not be getting enough fuel.