You should read the section on gas safety before attempting any repairs.

The gas valve is a relatively simple device. When the handle is in line with the valve, gas flows through the valve. When the handle is cross ways to the valve, there is no gas flow.

The heart of the valve is a cone shaped object with a hole through it that is fitted into the valve housing. The hole in the cone is in line with the valve handle. The cone is held snugly in place by a spring loaded tension screw passing through the valve housing into the cone. The wall of the cone is covered with a thin film of gas cock grease which prevents gas from seeping pass the cone.

Sometimes a foreign particle can wedge itself between the cone and the wall of the valve housing causing a leak through the valve and/or around the valve handle and tension screw. Sometimes pushing the tension screw, and therefore the cone, several times will clear the valve. Only do this when there is no flame nearby, because gas is released. If this doesn't work, remove the cone and clean it and the valve housing with a soft rag. Reassemble with a new film of gas cock grease which can be purchased (or borrowed) from most propane suppliers.