When it's good and warm out,get inside the truck,cover your face and begin to push it out with your feet.
The glass will break some more(o.k.,a lot more!),but it will come out,even if a piece at a time.
If you find any cracks in the rubber at all,you might want to replace it,since it will not be flexible enough to install the new glass without breaking it.
Try to lay the gasket as flat as possible until you use it. Spread it out somewhere flat, on the hood, bed, bench top, roof. Used gaskets aren't as pliable as the new ones and they need all the help they can get.