Could be the temperature selector. Or could be something else. Here's how you can check most of the parts. (This is how you do it on an electric dryer; it's probably about the same for a gas dryer.)
Unplug the dryer, and remove the back cover. Using a multimeter, test each of the electrical components one at a time, removing the wires before testing. I suggest using an analog multimeter (the one with the needle) - they are very simple to use and understand. If the multimeter needle moves, the part is likely good. If the needle doesn't move, the part is likely bad. Replace the bad part.