The first thing I always wonder about is what is it you are producing gas for. A vehicle? a Generator? One is going to require a lot more wood than the other. When I was still experimenting with wood rather than charcoal I built a wood dryer.
It would dry about 20-25 gallons of wood chunks down to about 20 per cent after feeding the Rocket heater for about three hours, but this was not sap wood. It was wood that was already seasoned for a few months. Still it was a pain in the ass to load and unload and I eventually got tired of messing with it because the rocket heater required constant feeding. Just mentioning it as an option.