I'm not sure the gasket is available based on reading a post elsewhere from Ken at 9E that they make a bespoke gasket in-house.
You might be better off giving the job to 9E or exploring that possibility with them. I'm not certain they sell the gasket to external enquiries.
Also the tip box can develop oil leaks in other parts and 9E do several fixes fir tip box oil leaks as well as changing oil and filter for the tiptronic box which is not as straightforward as it sounds. I had the tiptronic box oil and filter changed in my Mercedes a couple of years ago by the Main Dealer at a cost around £300 and around 2hrs labour - they reset the gearbox ECU, examined the old oil for debris and any smell of burning from the friction plates, new gasket for the sump and its a two-stage fill process completed with engine at running temperature. On the MB box, it is recommended to use only MB transmission fluid with several disasters cited on the MB forum from people who used generic ATF rather than buy it from MB on cost grounds. It maybe hog wash but even MB dealer had said they had some boxes in the workshop that had non MB ATF fluid and had to pronounce the box as unserviceable to some unfortunate owners.
This is with MB tiptronic box in MB W211 and may not be relevant to the MB-supplied tiptronic box for Porsche.