Registered in 1990 is different from a 1990 Model Year. I own a 1989 S4 Manual, which was the only model year when you could have a 928 S4 Auto, 928 S4 Manual - 2.64 final drive and 928 GT (also Manual) but with a different final drive 2.73, more powerful engine and a lighter aircon compressor, and sports suspension as standard. The GT also had a shorter throw gear lever. It happens that I own a 1990 GT too
My 1989 S4 Manual was manufactured in Nov 1988. My 1990 GT was manufactured in Dec 1989. Porsche's Model Years run Sep to Sep. Thus a 1990 Model Year car could have been made between Sep 1989 and Aug 1990. My GT3 is a Dec 99 car, but on it's CoC shows as year 2000... To add more confusion, Porsches weren't exactly flying out of the door at the time. It wasn't unusual to have cars like yours being registered late. There are some supposedly 1992 and 1993 S4/GT cars out there. Porsche officially discontinued the 928 S4/GT for the 1992 Model Year with the introduction of the 928 GTS.
Going back to Maseratis - the 928 GT is the equivalent of the GranTurismo Stradale in the 928 line up thus it should command a decent premium over standard S4 Autos and Manuals...
And yes, the GranTurismo is the true successor to the 928... Shame that Porsche never continued with it, but chose to turn the 911 into a 928...