torque = force x length

gravitational force as madires stated = m.g (mass x gravitational acceleration) = weight.

you need to breakdown your system into several simpler parts so you can determine their centre of gravity (COG) one by one. from there you can get length from COG to rotor and perpendicular length to vector of gravity. multiply weight with that perpendicular distance, add them all for every breakdowned parts (additive if same rotational direction, subtractive if counter rotation) you'll get total effective gravitational torque... well, for longer answer... attend a class.