I have had my GMG DB for 4 or 5 years now. I started getting a lot of soot every cook a couple of months ago. . I replaced the combustion fan assembly gasket and that solved the problem. The original gasket was burnt to a crisp. I used Hi-temp silicone with the new gasket. Works like a champ now!
A new gasket is 95 cents from GMG. If you are still under warranty I'll bet they will send you a new one free.