Drain Gear Box during oil change?

I do not think you should do that. When you burp the engine you pull the majority of the oil back to the tank. Follow the oil change procedure in the maintenance manual line in my opinion. 

The engine is dry sump and the gearbox is a shared oil system, there is no separate oil in the gearbox and no drain.  Oil from the gearbox is pushed in by the crank end and the circulation draws the oil out back into the crankcase were it is pushed to the oil tank with the rest of the engine oil.

Cheers

The small amount of oil left in the system during an oil change is no big deal. It's no different than when you do an oil change on your car. You just drain it and add the new oil. You don't flush the left over oil out of your car.

If you drain the oil out of the hoses on your plane or the oil cooler then you'll definitely be doing an oil purge because you'll be injecting all that air. So just do a normal oil change. Your dry sump engine is an easy oil change.