If your gas dryer is not heating up, then it's possible you have one of those parts that is not working properly.
First we start with the glow bar igniter.

How to test your gas dryer igniter?

How to test your gas dryer thermal fuse?

How to test your dryer high limit thermostat?

How to test your radiant or flame sensor?

How to test your gas solenoid coils?

How to test your timer or electronic control board?