I think this is what Beebob was saying, but when I've removed the glass, the glass stays in the gasket the whole time.
So, step 1: Remove gasket locking strip
Step 2: Get hot soapy water all over the gasket material.
Step 3: Remove glass with gasket still on.
Doing this in bright sunlight can help keep the gasket pliable too. Sometimes, getting inside the truck and running a flat-blade screw driver or similar implement between the gasket and fiberglass will help to get things started. Soapy water in there is also helpful.