Probably it is burning oil.
It happens if you a using a mist system of lubrication. If not, you should probably replace your piston rings. Combustion of oil causes fouling of the spark plug.
There could be many problems related to smelling gas and seeing white smoke from your tailpipe. Some suggestions would be the head gasket. If the smoke has a blue tint to it, it may be oil related. If your car has a turbo, it may be the turbo needing to be replaced. To be sure, you can have a mechanic do a diagnostic on it.