When you first turn on a gas burner you'll get a brief smell of gas until the burner ignites but then it should go away. If you're in the next room, it's possible you're smelling the residual gas from when he first lit the burners.

You need to be more specific about when the gas odor occurs. If it is right when the burner ignites at the beginning, and dissipates after a minute or two, then I agree with Weissman, and it's normal and not a big deal. However, if it is an ongoing smell, during the entire time the burner is lit, you may have a leak somewhere that you really should get repaired as that would be quite dangerous.

This is happening intermittently with our new thermador range when a burner is turned on. We have had the unit and have been cooking on it for three months and this has never happened before, then we cooked and the strong gas smell happened with one burner on. The smell permeated the entire downstairs (2,000 SF) of our home. When we cooked on it for the next three weeks, no gas smell and all and then tonight we cooked and strong gas smell from a different burner. We have opened all windows and doors. It was that strong.