Hi I have the same issue with my 2019 a4 b9, when doing the oil change I mistakenly removed the same drain plug, it’s the same one as your photo. I quickly put the plug back in, only losing about 300ml of fluid… after finishing up the actual oil change I moved the car off the ramps, and ran it while resetting the oil service light. Drove it around a couple miles with no issues. Since the gear oil is about 3.5L total to drain, as well as the transmission DSG, Is this a big issue? What did you end up doing to resolve it on your end? Thank you