How do I draw moments on hinged gates?

When the gate is drawn fully closed, it is understood that the system is in structural equilibrium with the condition $sum M_C = 0$ is true, so there is no moment to show on the diagram.

If a mechanical device is required to assist the hinge to resist the rotation due to the hydrostatic pressure, then its presence must be indicated on the hinge point "C" with the direction in agreement with its purpose. In this case, the same direction as rotation produced by the force "P".

On the diagram above, it is understood that the rotational equilibrium is maintained thru $M_P + M_R - M_W = 0$. ($M_P$ & $M_W$ need not shown though.)