Hi Edward,
I think the issue you are facing with the gas dryer is the lack of "make up air". This issue is often overlooked in dryer and other vented applications like stove hoods and bathroom vents. The issue is that these devices need to receive enough air to work properly, and if the house (or closet) is very air tight (which is the trend), then the dryer will starve for air. The result is incomplete venting of the dryer exhaust. This is less of an issue in an electric dryer as there is no combustion. But in a gas dryer, you will need to provide some make up air. One solution is to just open the window. But a better solution is a vent with a flapper on it that allows air to enter when the dryer begins to run.