There are other possibilities,

Weak igniter, sticky radiant sensor, or bad thermostat,

But your description leads me to believe your coils are weak.

Here's a way for you to check,

On the front panel of your dryer, there should be a 1x1 inch plastic cover,

You can remove that swatch of plastic and start the dryer,and observe the gas valve operation.

A few seconds after you start the machine, you should see an orange glow from the igniter.

After the glow starts, in 15 to 20 seconds you should hear a click, the igniter glow should start to fade, at the same time you should hear and see the gas released and the flame ignite.

You may have to observe the gas valve operation for several cycles.

What will probably happen, after several cycles, the igniter will glow, you'll hear the sensor click, the glow will diminish, but there'l be no flame.

If that occurrs, order and install the gas valve solenoid / coil kit.

Good Luck,
:) :) :)