How to troubleshoot Gas dryer not heating?

The most probable thing is going to be the igniter that is the problem. Gain access to the front of the dryer by removing front panel.Use a volt/ohm meterset to measure ac votlage and check were it plugs into (not the igniter but were it plugs into) and start the dryer and check for 120 volts into the plug..if 120 volts at that plug the igniter is bad and it needs replacing. Here is what the igniter looks like.
If it does come on when starting the dryer before unpluging igniter to check voltage,watch and see if their is a flame..if their is a flame watch it a few cycles and see if a few flame on flame off cycles see if a couple times after coming on if the igniter comes on and goes off but no flame the gas valve coils will be the problem. Here is what the gas valve coils look like.

if the igniter does come on but not go off followed by a flame the flame sensor is going to be whats needed its pictured here.

If NO voltage to igniter OR it doesnt come on,what i would suspect wrong then is either the motor or control board(or timer) being the trouble. However these things i covered are the most likely things to go out