As there is no requirement by Vauxhall to change the gear oil as part of a service, there is no actual drain plug provided.
At the end of the gearbox, there is a metal plate held in place by 10(?) bolts.
This plate needs to be removed in order to allow the oil to run out.
You could, I suppose, simply slacken off the plate, but then you wouldn't know if the gasket had remained in tact, in any case, it is safer to fit a new gasket than to risk an oil leak.
When tightening the plate back up, work evenly, tightening a little at a time, working at opposite sides of the plate.
That's because there isn't one. Only way to drain it is to loosen those 10 bolts around the bottom plate and let it drain out into a pan as much as possible before removing it entirely. It then needs a new gasket when the plate is replaced.