Hook tool, cotter key puller or radiator hose removal tool. It has a sharp point, the hook tool with a ball at the end is used for cleaning temper glass piece out of the run channel when you break a door glass. The rubber gasket is pretty tough, I would be far more concerned with you wedging the tool in there and breaking the glass. You allwise should be pulling away from the glass. Look up equalizer fiber stick. It is what I use. I usually reserve the hook tool for when things get tight/hard, but you have to be more careful that you don't break the glass.
They should have used a non hardening sealant between the gasket and body to prevent leaks. Foaming glass cleaning makes a good line. You can also push the lip of the gasket out from inside the truck as someone else works the edge up around the glass in tight areas.
My first windshield was a late 60's dodge truck with the high sharp top corners. Didn't know what I was doing , using screw drivers. I put so many chips(shells) in the edge. To this day I am still amazed I didn't break it.

Good luck.