Looks like a good buy.
My Buick parts books shows the 65 GS uses the same axle and rear wheel bearing as the 64-65 Sportwagon, so it should be the HD version. Double check to be sure..... I don't believe the Special station wagons got the HD rear, just the Sportwagons!
BTW, the '64 used a small diameter upper control arm bushing. Expect to pay more for replacements.