The gas going into the engine ideally is broken down to hydrogen and carbon monoxide, with some methane. This is also scrubbed of all tars and ash for the safety of your engine. Within the pistons of your generator's engine it is combusted to water and carbon dioxide. Unlike gasoline and diesel which needs to pyrolysis within the engine's cylinders or fuel rail, the syngas has already been broken down to its bare combustable components...so emissions are usually much lower.
That being said, there is a variety in the solid fuels used, and operational and engineering problems to overcome for some waste streams. There are backyard versions that use water to scrub the woodgas, leaving behind large ponds of toxic waste full of PAHs (Poly Aromatic Hydrocarbons) and other carcinogens. But the exhaust from the motors is generally better than that of a gas or diesel engine. (Source, I used to work for All Power Labs, where we did a variety of tests on our fuel, exhaust and ash production)
That being said, there is a variety in the solid fuels used, and operational and engineering problems to overcome for some waste streams. There are backyard versions that use water to scrub the woodgas, leaving behind large ponds of toxic waste full of PAHs (Poly Aromatic Hydrocarbons) and other carcinogens. But the exhaust from the motors is generally better than that of a gas or diesel engine. (Source, I used to work for All Power Labs, where we did a variety of tests on our fuel, exhaust and ash production)