Gear Lube and Gear Oil are pretty similar but if you have gears that are hypoid gears then you need gear lube instead of gear oil.
Gear Oil can be used in almost anything else that requires Gear Oil or Gear Lube.
SAE 40 or 50 motor oil also is the same viscosity as SAE 90 Gear Oil so that motor oil can be used where something calls for g ear oil of SAE 90.
Motor Oil just contains additives to protect engines.