Gear oil is thicker than hydraulic oil since the gear oil is needed to stick more to the gears to lubricate gears such as in differentials.
Hydraulic Oil is thinner so that it can move through a hydraulic pump and put pressure on hydraulic cylinders to move the hydraulics in and out through pressure.
Hydraulic Oil and Motor Oil are not very different and adding some motor oil to hydraulic oil is usually safe depending on the machinery you're putting it into.
Gear Oil is too thick so it shouldn't be used as hydraulic oil and hydraulic oil is not thick enough to lubricate gears properly so hydraulic oil shouldn't be used as gear oil either.