Warning : Gas systems are generally safe but any gas smell should always be investigated by a certified technician as you can never be too safe.
That being said, let me try to assist you as best I can from my experience with gas appliances.
If the smell has been there since you bought the unit, it is probably the pilot you are smelling. Generally, a pilot light is first ignited using either an electro-mechanical switch or using electricity to create a spark.
While ignition is taking place, a small amount of gas is allowed to flow through the ignition pilot nozzle. The spark is meant to ignite the pilot flame. When the pilot flame is lit, a thermocouple is heated up and then allows gas to flow to the main burners. The gas in the main burners then ignites directly from the pilot flame. This is generally how gas burners work but there are many types of equipment and not all of them work on the same principles.
Generally when you smell gas before igniting the main burner but detect no smell after the main burner is on, it has to do with problems in the ignition stage. This could be caused by a number of things but often the ignition probe is the problem. The ignition probe arcs a spark between two probes or between a probe and the burner which is meant to ignite the pilot gas nozzle (or in some cases ignites the main burner directly). Everytime this spark is made, a small amount of carbon forms at the connection point of the sparks. The carbon can build up over time and cause difficulty sparking the ignition of the pilot burner. Spilled food can also cause this or speed up the buildup of carbon.
If the gas can't ignite the pilot immediately, the pilot keeps allowing gas to pass into the air until ignited which is what you could be smelling. Check to see how long after you try to start the burner does it actually ignite. More than a couple of seconds and you will be smelling the gas that escaped without being burned off.
You can try disconnecting the unit from mains power and cleaning the ignition probes on each burner. If the unit has a mechanical starter you should take extra care not activate it during cleaning as you could get a electric shock. Clean the probe with a damp cloth or paper towel and don't use any cleaning agents.
If this does not eliminate the smell you are getting before ignition, you should get a technician to test the unit. The explanation above is also very general as I don't know what specific system you are using so use the explanation given as general knowledge only and get a technician to perform the repairs.