You didn't include a actual model # but I'll give you the general rules of gas dryer diagnosis.
1) If it heats at all your thermal fuse is fine.
2) Verify a strong blast of air from vent to outside.
3) If 1 and 2 are ok it's probably your valve coils, as they fail they begin to allow the valve to close prematurely and you get low temps. Usually this happens progressively as the load runs; the first few times you get a good burn but subsequent burns are shorter and shorter. If it is older than 10 years they are a likely suspect.
Coils are a very easy DIY fix. Another sign of failure is a distinct buzzing during the burn.

This is an older machine, if there a little plastic cover down low on front panel? It pops out to view the burner in action. If not you can open the top and see it light up below. I'd need you model number (inside door probably) to be more specific as to how to do these steps.