P700walk,
Not sure I can help based on your description. I do not have anything I would call a "gasket" on the sides of my sunroof ('97 960 wagon) The only gasket I have is on the back edge to keep errant water from splashing foward onto the sunscreen. This keeps any water in the rear channel and forces it to the rear drain holes on either side. You can reach that one if you tilt the roof backwards and reach in - or if you take the roof off.
If there is or should be some rubber on the sides where the sunscreen slides in the channel, my guess is that it is just to keep the sunscreen from rattling.
I've never tried, but I am sure I have read that the gasket on the glass itself cannot be replaced and you have to buy a whole new piece of glass. I keep mine dressed with Silicone lube to keep the leaks away and have also read here about using a Butyl (sp?) rope tucked under the lip to seal the top edge of the glass.
Sorry if that's no help - and good luck,
Mike