Engine runs hot/coolant smells like gas/no heat?

2000 Cadillac STS, After 2 months of driving perfectly, yesterday the oil Temperature indicate started flashing and the cars computer stated that the car coolant was too hot. After further investigation of the coolant, Ive noticed the color of the coolant is darker than normal and has a gas smell with a sludge/rust substance on the cap covering the coolant system. I wil try flushing it today. The car starts fine and it doesn't overheat but it runs very Hort, reaching way past the middle of the Temperature gages found on the dash board. I tried turning on the heat and it only blows cold air with no smell? Any suggestions on what i might be dealing with?