Try replacing the thermal couple (milivolt generator) that the pilot light heats up. If it is going bad the pilot light will not stay lit or it will not allow the gas valve to operate. It is cheap and quick to replace it , about $20.
If the pilot light is staying lit try jumpering around the flow switch and on/off switch using a piece of wire. Make sure the pilot light is lit and quickly bypass the switches by touching a wire between the two terminals on the gas valve that the switches are wired to.
Be careful if the gas valve is under the burners. Hold the wire there for about two seconds, the burner should light. If it lights then your flow swith or on/off switch is bad.
If it dosen't light replace the thermal couple. If it still dosen't light after replacing the thermalcouple then you have a bad gas valve.
Dave