In which case please bare in mind I use a wheel so my advice may not quite work for you. In short, on lose surfaces like dirt and gravel I set my transmission up much the same as for snow and ice. A very short box allows you to keep the boost on bit means you change hears a lot. This is useful because the engine braking can be used to affect the dynamics of the car. Helping both to spin the car a little and to catch inadvertant spins. Fairly strong diff settings are required but not so strong the car becomes unmanageable. I usually lock the diffs and shorten the box radically and get on the practice tract, winding back the settings as needed. Good luck.