My husband had a gas can for our snowblower stored under an overhang on our house, the can had a hold in it and the gas leaded out going uner the house, there is a strong smell of gas in our house what can I do? How harmful can this be?

Well Gasoline is strong stuff so it is not good. I am assuming it was 1 to five gallons of gas. If it is really strong through out the whole house I would leave the house and call the fire dept and they will probably wash down under your house with their hose. If it is just in one room I would close off that room and rig up a fan in a window (using cardboard around the fan to seal it off) and use it to vent the odor outside. It is very important to ventilate the house. Gas vapors are extremely volatile and can quickly build up to explosive levels. Even in cold weather gas evaporates rather quickly. It should start to dissapate soon.