Whilst you can choose which fuel type you would like to cook with, we would not recommend using both gas and wood together for a number of reasons.
Firstly, wood and gas burn in a slightly different manner, meaning combustion of both within the same space can cause competition for oxygen within the chamber and a less than ideal convection of heat.
Secondly, the gas burner head will be exposed whilst in operation and during this time, there is potential for ash and debris from a wood fire to fall into the burner head and block the gas burner jets.
Finally, when running two fuels at the same time, there is a possibility that the oven may creep over the ideal operating temperatures. Whilst this is unlikely to damage the oven in any way, the large amount of insulation within the Master may mean that it takes some time to drop back to ideal temperatures.