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Traditionally, stripes on the fendor (and on the rear bumper) meant "rookie". It let other drivers racing against you know as they were approaching. However, GM took it and just made it "Grand Sport" it has no other meaning for GM. Other manufacturers adopted variations of stripes for special models over the years as well.
FYI, it is a dealer applied decal - so not all Grand Sports will have the stripes if the owner opted out. Likewise, you can order them, and apply them yourself to any make/model. So just because you have stripes, it also does not mean you have a Grand Sport necessarily.
During some races of the original 1963 Grand Sports, the teams painted one, two, or three stripes (or placed strips of tape) on the driver's side fenders instead of livery numbers to identify the cars. The hash marks on the fenders of current versions simply commemorates the markings on the original race cars.