If the dryer doesn't heat, but the drum turns, check the thermal fuse(s). The thermal fuse protects the dryer from overheating and helps to prevent fires. If the thermal fuse is blown, it will have to be replaced.
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.
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.
High Limit Thermostat
Although not common, if the dryer doesn't heat the high limit thermostat can be defective. It can be checked for continuity. If the thermostat is "open" the dryer doesn't heat.
The cycling thermostat regulates the temperature of the air in the dryer. If the dryer doesn't heat it may be caused by a defective cycling thermostat. This is not common.