Truly and orphaned brand. Makes even more sense that Richard Hammond named his Opel 'Oliver'...70-80k miles is about how long my Solex lasted until it wouldn't idle at all. I was really busy at the time and the GT was my commuter car, so I put a new weber on and never tried to resolve the Solex problem. The Weber ran good right out of the box.
Another reason that so many of these cars died at around that mileage was that Buick stopped selling them in the mid-late '70s, then stopped working on them in their shops, then stopped supporting them with parts by the late '70's or early '80's. There was a period there when you really had to scratch to find parts for them, so many GT's just got parked out back and forgotten at that mileage because it was just hard to keep them running then.