You have most likely blew the output seal on the gearbox and that’s why the oil is leaking. This usually caused by the vent plug being blocked or obstructed in any way so the gearbox as it heats up is not exhausting the pressure build up due to heat and the weakest link ie. the seal ends up giving way and you get a leaking gearbox. Best thing to do is to ensure the vent is fully cleared, replenish the lost oil and get a replacement seal. If that doesn’t work then you may have some more serious problems.