My memory of this gear is faint and blurred. I think (?!) that I have heard two stories about it, neither of which is supported by any documentary evidence.
The first story is that SunTour did not want to release their sealed parallelogram design as a part of the ‘Superbe’ groupset - as it did not have the signature detailing (sealed-bearing pulley wheels and stainless steel small parts) that they were building into their top line groupset. They originally released it in Japan as the GranTech. But cold hearted and grasping American marketeers forced them to rebrand it as Superbe Tech to provide an ‘ultimate’ groupset to pander to the booming mountain bike market.
The second story is almost the opposite; it goes that following the debacle with the radical Superbe Tech family of derailleurs (see the SunTour Superbe Tech L (4800) and SunTour Superbe Tech GTL (5400)) SunTour was in something of a bind. It still had some faith in the short cage Superbe Tech design and it probably also had a huge capital investment to pay off and some serious inventory to clear, so it did not want to ditch the project entirely. But it was also desperate to avoid further tarnishing the name of its cherished ‘Superbe’ groupset. Time for some rebranding - as the SunTour GranTech. The only technical difference that I can see is that the hollow hanger bolt has gone and an interesting captured hanger bolt has been added - just the sort of change that a bike manufacturer would request to make assembly easier. Perhaps SunTour decided to clear its inventory of Superbe Tech parts out to a chosen group of lucky Japanese bike manufacturers - who undoubtedly got a bargain.
I’m guessing that the second story is true (because I seem to remember seeing these after the Superbe Techs). A sad end to a brave but foolhardy project.
The GranTech, hmmm, on the one hand it calls to mind the glories of the Campagnolo Gran Sport and the SunTour Gran Prix, but on the other hand you can’t avoid the terrifying ghost of Thora Hird on her (misidentified) Stannah Stairlift.