I would check the thermal fuse first, part # 3392519
On a electrice dryer if the thermal fuse is bad the dryer will not start, but on a gas dryer it will start but not heat.
If the thermal fuse checks ok then I would suspect the igniter, part # 279311
. I would check to see if voltage is getting to the igniter. If you have voltage to the igniter and it does not glow replace the igniter.