[Gelbster]

First you need to clean everything.It will take a loooong time. If it isn't clean ,how will you see where the leak begins ?
Is the vent partially blocked ? Is the oil level overfilled ?
When was the gbox oil last changed ? Oil change for the 6-speed is often neglected.
If you find an oil change is overdue, do it because this is a very expensive gearbox(6 speed). Use correct(obscure) oil(Search).
Whichever box it is, do the 'sticky finger' test in the drain hole to find the magnet & sludge/grease - if any. Sludge/grease=imminent bearing failure- so you have a bigger issue than a little leak . Lots more in Search. The 6-speed uses some sealed bearings like the infamous IMS. If you catch the issue early ......