Snyder's catalog says "This gasket goes between the face of the tail light rim and the lens, but some people put one on each side of the lens to help avoid cracking the lens."
The part that Snyder's is calling the "tail light rim" is called the "Rear lamp door" in the Parts Price List book.
The gasket is made to fit between the rear lamp door and the lens. The gasket has the same divider between the lens segments as the rear lamp door has. If you put the gasket between the lamp body and the lens, then there is nothing to hold the divider in the gasket in place and it might eventually sag. I would suggest that if you put a gasket between the lens and the lamp body, then you might consider cutting out the divider part of the gasket.