If you want to reuse the glass, the best way to remove it is to cut the seal around the perimeter of the glass. There is a groove for the glass, and another behind it that goes over the pinch weld on the body. The gasket itself holds the glass in place. There's a 50/50 chance whether there is any sealer glueing the whole mess to the cab, the gasket turns to wood over time anyway and with or without adhesive it's very tough cutting through the gasket. A soapy lubricant may help get the blade through.
If you have stainless trim around the glass, do not attempt to remove it before the glass. The trim is inserted in the gasket, then mounted on the glass, and the assembly is then installed in the truck. The trim must be removed from the gasket after the glass is removed from the truck in order to avoid damage.
