Don't know about the gas smell in the vents (that sounds bad), but you should try to visually locate the leak. Draining coolant could mean a blown headgasket, a cracked radiator, cracked or leaky expansion tank, bad coolant hoses or just worn hose clamps, water pump, thermostat housing, or even just a worn or cracked expansion tank cap. (did I miss anything guys?)
The problem with a coolant leak is that it typically will occur only when the system is hot (under pressure), so that means, on the road. The coolant will evaporate before you can find it.
Check under all the hoses and the bottom edge of the radiator for dried coolant. If you don't see anything there, the leak could be inside the block. I have a bad water pump & hoses (still waiting for parts to come in); I was able to visually locate the leaks' origin at the junction of the radiator-water pump hoses with the water pump (as well as a few other small leaks at hose connections) from beneath the car.