I was concerned that it may have been an olds rear.
nope, the third letter would be an "O" if that were the case.
what i mean by 'bad news' from the chassis manual is that a 350 GS would vin code as a Skylark, the Gran Sport would just be an option package on top of that.
the problem from the chassis manual is that there is only one "economy" option ratio shown for a Skylark / Custom, 2.56:1. "LKB" for peg leg and "LTB" for limited slip.
the LOB 2.73, according to the book, was ONLY supposed to be in the Sportwagon. unless the GS package was also an option on the Sportwagon, that rules this out as an OEM 'GS' part ... unless you can find an option sheet showing otherwise.
this was before a lot of computerized automation and a lot of strange things happened that weren't necessarily "by the book".
but you've already told us this is a replacement rear anyways. just don't try to justify it as 'original to the car' and i doubt you'll have any problems. externally and even parts wise, this rear is identical to what came in the Skylark / 350ci, it's just the gear set and tube stamping that's 'wrong'.
most likely, the rear will never get looked at unless you're trying to win a concours show.