If the problem with your Whirlpool 29 inch gas dryer is that is not heating at all, the problem could a bad flame sensor.
The flame sensor is used to turn the igniter on and off.
This switch has two bimetal strips inside of the switch with contact points. One of the two bimetal strips is made of two different metals. When the strip is cold, the contacts are closed and the switch is on.
When the heat from the igniter and the flames heat up the metal strips, the metal strip with different metals bends, the contacts separate from each other and the switch is turned off.
When the cycling thermostat turns the flames off, the strips cool off and the switch gets turned on again.
Below are the links to the parts needed for this repair.