You can't buy a new casing it's the whole gearbox assembly...
There is a specific TPI for the leak, and I procedure to follow to fix it.
In my instance the procedure was carried out twice but the crack in the case in was too big and it still leak so VW authorised a replacement.
I'm not saying that's what yours is you need to clean all of the oil off to then find out where the fresh oil was actually leaking from.
Could be coming from the oil cooler or it could be coming from the known issue with the DSG box where it could leak from the bolt hole... Or possibly be coming from somewhere else?
But the worry now is how much oil have you lost? Ours was recovered straight to Volkswagen under the all-in warranty as soon as the leak.