I am not in the business of fixing gas-leaks but my father is knowledgeable in that area. He basically said that you would have to go to the place in the house where the smell of gas is most evident (strongest) and you have to check the pipes.
There are some sort of "detecting solutions' you can purchase to put along the pipes and joints of pipes ... around the furnace, stove etc. If you do not have cash or time to go in search of such a solution, you can make it yourself with soap and water. When you put it along the pipes, if there is a leak - the soapy water will bubble-up. The liquid solution must be concentrated though - not watered-down.
If you need more specific information you can contact me through email or I am sure other people on Hub-pages will have better answers and opinions.