If I consider that the exhaust duct of the gas burner might be clogged with a bird's nest (and I know from my own experience that this is not absurd - at our place not only was there a bird's nest in the air shaft, but an entire crow was sitting there and had to be rescued by the fire department), I'd rather let the chimney sweep earn his money. And all these smoking chimneys around here - where people have switched to wood burning - should probably also be properly maintained.
I think Mr. Theisen should rather switch to tilting at windmills.