Well.... actually, no. Not really. All player gravitiy codes work with c_trace in some way.
There are ways to use the physics engine, as darkinferno said, tough... He just doesnt want to tell us.

afaik, the basic method is to reset the players angles every frame by disabling the physics for one frame, setting the angles back, and that enabling the physics again. BUt this has a certain effect on the frame rate tough.