forget any regular German NISSAN-Dealers as they do not have any access to SKYLINE-data as this car was never designed to be on German roads. So it is to be expected to pay way on TOP when asking for service.
Anyway, there are several Japan-Car workshops around Germany but most of them are private Backarea DIY´s of ethusiasts who are very helpful and some are very recommended, but when it comes to warranties, it´s somewhat tricky.
I did import a R34 "GT" in 2008, just an ordinary non at all modyfied version and was gobschmacked what prices came up when trying to get it road legal in Germany (MOT´ed)
Anyway I was lucky enough to find the company "MPS-engineering" and must say, that with my 46 yrs of age having seen many car-wokshops (primarely Nissan-Dealers), I surely consider "MPS" my most favorite for Japancars, especially SKYLINE´s
MPS is located in DÜLMEN (Duelmen-Muensterland Northrine Westphalia) where also the WIESMANN-Roadster manufactors are.
MPS helps Briton with a used R33 GTR gearbox:
I met Justin the R33 GTR owner last year Carfriday at the RING, he was on a bike and when he came to our parked Skyline´s he did ask for help regarding a gearbox as his gave up on the track. He was stranded already a few weeks at the RING as no British dealer was able to come up with what he needed. He was given many adresses during his stay but when I had Justin call MPS the next day he was able to pick up his exact needed gearbox for an outstanding low price.
PS: though my car has MPS decals on it but only for the benefit to have reasonable service as I am a gardener-landscaper with only a green and not oily thumb !!!