Gas Dryer not heating up
A dryer relies on both heat and air flow to dry clothes, and if the air flow is restricted, the dryer may stop heating altogether. That's not the only reason why it may stop heating, however. A gas dryer relies on a number of electrical devices to ignite gas and regulate its flow, and if any of these are defective, the tumbler and fan will work, but the dryer won't get hot.
If the air flow in the dryer is restricted, the temperature in the heating chamber can get hot enough to blow the thermal fuse. If the fuse blows, it cuts power to the coils that control the gas valve. The fuse is usually mounted to the exhaust duct just inside the back panel. You can check its continuity with an ohmmeter. If, after disconnecting the fuse, you get any reading other than 0 when you touch the leads of the meter to its terminals, it has blown. There's no way to restore it - you'll have to install a replacement.
Igniter and Gas Valve Coils
The igniter is an electric conductor that works like the element in an electric heater, glowing hot enough to ignite gas when you turn on the dryer. This conductor can burn out, and when it does, it may glow, but it won't get hot enough to ignite the gas. Occasionally, the electric coils that control the gas valve are defective - they can wear out when the dryer gets old. When this happens, the igniter glows, but gas never enters the heating chamber. Performing a continuity test on either part will help you determine whether or not you need to replace it.
Air Flow and Heat
The motor that drives the tumbler also drives a fan that circulates air through the heating chamber and the tumbler and expels it through the vent. If the air can't circulate, perhaps because of lint blockage, the heating chamber overheats, which prompts the cycling thermostat to turn off the gas. The thermostat resets when the chamber cools, but the chamber heats up quickly and the thermostat again shuts off the gas. The result is that the temperature in the tumbler doesn't stay hot, and your clothes take longer to dry, if they get dry at all.
There's a big difference between a dryer that doesn't heat up at all and one that just doesn't get hot enough to dry your clothes. In the first instance, the problem is usually a defective part, and things should be back to normal after you replace it. In the second instance, the problem is caused by restricted air flow, and you need to clear the lint filter and vents and take steps to prevent lint build-up. If you don't, you'll use more energy for drying than you need and the dryer may continue to malfunction. Worse, you may have a dryer fire.
Why is my gas dryer not heating up?
It could be caused by a damaged heating element. You should have the heating element checked by a professional. Try cleaning the dryer first. Remove lint clogging the exhaust vent. Overheating in the washer might cause the heating element to shut off safely. Also check if the dryer is plugged correctly.