I agree, cutting of the gasket MAY not be necesary. If its old and brittle, of course you will have to cut it, but you wouldn't even want to reuse one in that condition.
I needed to replace the glass in my '82 300SD that I had removed for painting. I cut the gasket on that one so I wouldn't distort the trim. It distorted anyway, and would not go in. I ended up cracking the glass trying to make it all fit, including a new seal.
Then I went to my local auto junk yard. their fixed preice on rear glass is $40 if you remove it. Heck, I wanted to remove it myself. I found a later W126 with a good seal; no crack, and quite supple. After removing the trim and unscrewing the heater connections, ran a flat-bladded screwdriver all around the inner edge of the seal to break the bond. Then I pried one of the upper corners over the lip in the opening, and worked the edge across the top of the glass, prying a little and pushing a little, Once I had a good start, I put my feet on the glass, right at the edge, and pushed while reaching past my feet to pull the edge of the seal over the lip. After a few minutes, it just sort of all unzipped. I ended up with a good glass, with the seal and trim intact. I reinstalled it the same way, never taking the trim out of its channel.
I reinstalles easily using a rope in the channel for the body pinch weld. Keep it well lubed. I also (gently) laid an 80 lb bag of sand on the lower edge on the trunk side to hold it in place while I zipped out the rope.