Something everyone here seems to know something about to some extent. To the best of my knowledge many, (myself included) have little to no experience with these seemingly wonderful little contraptions.
And the people that know are exceedingly rare to find.
Gasifiers have an interesting history, but I'll leave it to the professionals to tell you all about it.
The basics of it however are that the gasifier has been known about for just over 400 years, and in general use for almost 200.
And they work. In general a wood gasifier generates about 60 percent of the energy of gasoline. That means if you have a 10 Kw gasoline generator, it'll crank out about 6.5 Kw on wood gas.