Nothing in the VIN or on the trim tag will tell you if it is a GS, see this link for identifying a GS:
http://rivowners.org/features/Ev_Stats/66-69/index.html
494877H909xxx
4 = Buick
94 = Riviera
87 = 2-door sport coupe
7 = 1967 model year
H = built at Fint, MI
909xxx = sequential number, range was 900001 to 942811 for the Riviera
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