Gas Valve Solenoid
The gas valve has two or more electric solenoids which open the valve to let the gas flow into the burner assembly. If the dryer doesn't heat one or more of the solenoids may be defective. If the igniter glows for 90 seconds but the burner flame doesn't light, replace these coils as a set.
If the dryer doesn't heat, the igniter might be burned out. Check the igniter for continuity with an ohm meter. Dryer igniters burn out over time, similar to a light bulb.
Gas dryers have a flame sensor to detect the heat given off by the flame. If the dryer doesn't heat, the flame sensor might be defective. Check the flame sensor for continuity. This is not as common as a burned out igniter or thermal fuse.