First, check the vent to see if it's clogged or obstructed e.g. crimped flex hose, bird or squirrel nest, lint, etc. This condition can cause the drying time to gradually increase, and, as probable in your case, one or more of the thermostats to open.

Do you have a multi-meter? Can you check the thermostats?

There are a couple really common causes for no-heat problems on a gas dryer, and one that is less common: Check to see if the ignitor is glowing. If not, replace it (be careful not to contaminate its element by direct touch [similar to the caution in handling your car's headlamps]). If you see it glow, and cycle on and off, the coils should be replaced (their function is to open and close the gas valve).

The less common failure is the radiant sensor. Its function is to sense the glow of the ignitor and allow current to flow to the coils, so that the gas does not flow in the case of a radiant or flame failure.