A gas leak is different from the concern about byproducts of the combustion process. Natural gas is odorless so they add a chemical to it and that is what you smell when there is a leak. Carbon monoxide is one of the byproducts of burning fossil fuels. CO2 is what gives soda its fizz. A gas leak is a serious/urgent problem, can cause health issues and needs to be immediately addressed. If you smell gas in your home, you need to get someone out to find the source and resolve it. It can cause an explosion if enough accumulates in an enclosed area.