The VIN doesn't tell you if it is a GS or not. Would need a build sheet, etc. https://rivowners.org/features/Ev_Stats/65gs/ https://rivowners.org/features/Ev_Stats/70-75/index.html
The 1971 Riviera VIN begins with 49487. This was the same for all '71 Rivieras including the GS option. No way to tell from the VIN if it was a GS.
If I am not mistaken, all Rivs at least through the 4th generation had bodies built in Euclid, OH and shipped to Flint for final assembly.
Riviera plants 1963-1980: 1963-1965 body and final assembly at Flint MI 1966-1970 body at Euclid OH and final assembly at Flint MI 1970-1977 body and final assembly at Flint MI 1979-1980 body and final assembly at Linden NJ
So it looks like no chance of getting build info from GM of Canada in the case of Riviera like there was for the Skylarks which were built in Canada. Bummer.
GM of Canada documentation is available for any car built in Canada or sold new in Canada. So you can still get documentation for your Riviera if it was sold new in Canada.