This HAPPENS. Did to me on my 1988
Scamp. The gasket shrinks. I filled that little area (both rear and front plexi windows) with Black Windshield "goop." It's a caulk of some sort... NOT silicone (just like Jared mentioned), but that little gap can let in a bunch of water. Be neat, no need to blob it on and you'd be surprised how little you need to fix that gap.
In the end, you need to replace the gasket and lockstrip material. But my "quick" fix has lasted for a half-dozen years...