Hiya I retrofitted a tailgate to my t6 and had to guess where the hinges went as they weren’t supplied with mine, I found that adjusting using the bolts you can get to from inside the van copes with fine up and down adjustments. There is an official procedure where you disconnect the struts before you adjust the hinges, which I did, as the struts will try to force the tailgate up with the tailgate bolts loose.
I propped the tailgate open and removed the tailgate struts, closed tailgate, loosened the nuts inside the van allowing the tailgate to sink to its lowest point, then used wedges on the outside between bumper and tailgate bottom edge to eyeball it level. Once happy tighten up the hinges again, then lift up and prop tailgate to refit the struts. Hopefully goes without saying but the tailgate is v v v heavy and be very careful around it without the struts disconnected potentially lethal !!!
You will need a very deep 13 mm socket to fit the nuts on the hinges from inside, for reference there is a detailed barn door to tailgate thread with lots of helpful hints !
Good luck