GL gear oils 75W; he is right, has probably same weight as engine oil of SAE 10W-40.
Viscosity is not the problem here. It is actually additives in the GL gear oils which are not good for some gear components. These are corrosive for some brass based parts of the gear boxes. Hence you should use only the oil recommended by the OEM.
You cannot even use a GL-5 gear oil in a gear box where GL-4 is recommended. If Honda is not recommending a GL oil there should be some reason behind it. Using any other oil will damage the cars gearbox and a premature failure.