Tumbling fine no hot air. is a gas dryer

First, check that the ignitor is working properly. It should glow visibly if it is working. If the ignitor seems to be functioning properly, then I would suspect the gas valve coils. These electrical coils are right above the gas valves and when these solenoids are energized they open the gas valves and permit the burner to light. If they are faulty, the gas valves don't open so no gas gets to the burner. Due to the difficulty in testing these solenoids, it is recommended that both (all) be replaced at the same time if any are suspected of being faulty. Hope this helps and best wishes.