Gas leaks are extremely dangerous. They can lead to loss of human life and property. If you suspect a gas leak on your property, the first thing you should do is evacuate and call the experts. If there are any occupants present in the building, you need to alert them too.
If there is a gas leak inside, you should open all doors and windows to promote cross ventilation. The ventilation will allow the gas to escape and be replaced by fresh air. This is not a solution to the problem of a gas leak itself. It only lessens the chances of accumulation of the leaked gas.
If you have a gas leak in the garage or outside your home, you should not start any vehicle. A spark in the engine of the car can ignite the car. While there is less chance of this happening, it is important to take precautions.