How reversible is decerebrate posturing caused by brain stem damage?
The decerebrate posturing is a sign of major brainstem dysfunction.

The reversibility of this state (which is considered to be very critical and hardly reversible) depends upon the origin of the brainstem dysfunction.

The primary brainstem dysfunction means that the brainstem was directly damaged (by a bullet) and there is no way that this can be quickly reversed. This is exactly the case here and there are no chances that the brainstem might recooperate with time.

The secondary brainstem dysfunction may be caused by cerebral edema and thereby dislocation of the brainstem towards foramen magnum, where the brainstem is coned. The coning of the brainstem can be reversible if the brain edema is treated quickly and effectively (by administration of diuretics in the intravenal catheter. 

With brainstem regaining its normal functions the decerebral posture will eventuall resolve and depending upon the residual brain injury the recooperation of the brain function (and compensation) may be almost entire.