You need to see if the ignitor is glowing red which lights the gas flowing thru the valve. The front cover of the dryer may have to be removed to observe that if it does not have a small viewing cover on the front. If the ignitor is not heating it will not light the gas . A bad or broken ignitor will not let the gas flow. However as it heats up, a click is heard coming from the valve ,the ignitor turns off after the gas valve opens and the flame starts. If you are not familar with these parts or how they work together you should consider calling a serviceman.