From the description you give I suspect the problem will be with the gas valve or coils.
The difficult part of the problem is that it heats to start with but then stops heating.
There is a small square plug on the front panel that can be removed.
The key to this problem is knowing what the ignitor is doing.
If the ignitor continues to glow brightly, shut off and then glow brightly again then the thermostats, timer, thermal fuse and ignitor are all okay. The problem would most likely be with the coils or valve.
If the ignitor only glows once or twice then never again then the problem will be with the timer, thermostats, thermal fuse, ignitor or even motor switch.
It is not unheard off for the valve/coils to work a couple of times then stop working. It can be the coils or the valve. The coils can both be gotten for about $20 dollars while the valve will be closer to $100 dollars.
When the ignitor is glowing brightly then radiant sensor should open with a click and direct the voltage to the coils. If you a buzz or hum from the coils but there is not flame then likely the valve is at fault.
If the dryer did not work at all it would be easier to check.
While both coils individually cost close to $40 dollars a 279834 coil kit with both coils cost only $21.85 dollars and might be worth a shot. The valve is over $100 dollars and may not be considered economical.
The images below will so what is needed to access the valve.
Remember to unplug the dryer first.
You can reply with any additional details for further assistance.