Your gas dryer is probably only igniting the burner at the very beginning of the cycle, so when it first starts you will find heat inside the drum, but if you check it 10 minutes later, it will be cool again. The gas valve has two coils that begin to fail with age resulting in them only working a couple times at the start of a cycle, then they won't work again until they have a chance to cool down. Check to see that the igniter is glowing but not igniting the gas (there is a small square plug that you can remove from the lower left front panel and peek inside). If this is the case, then a couple new coils (Coil kit (gas valve) - 2 coils) should help.

The parts are easy to replace. The top can be removed by grabbing each rear corner, pulling it toward the front then lifting it up. The front panel is held in place by a couple screws on the inside, then lift it off the lower clips. The coils are part of the burner and you just need a screwdriver to change them. They only go on one way so it's not to bad.