Diagnosis
Restricted airflow. Lint is clogging the internal ducting, the external vent, or both. This is the #1 cause of long dry times.
How to Fix It
Disconnect the flexible duct from the back of the dryer. Vacuum out the dryer's exhaust port. Remove the lint filter housing inside the drum opening and vacuum there. Then, clean the entire external vent line from the wall to the outside cap. Use a vent brush kit.
If That Didn't Work
The gas igniter might be weak. It should glow bright orange. A dim glow means it's failing and not opening the gas valve fully, producing low heat. Replace the igniter (part DE93-00634A).
Heads Up
Never run the dryer with a clogged vent. It's a fire hazard and can blow the thermal fuse, causing a no-heat or no-power problem next.