That rear window is flat glass. Any automotive glass shop worthy of the name can cut a window to fit the opening in the cab, although I can't imagine why you'd have a window opening that's different from thousands of other Chevy and GMC trucks with the same cab. Are you sure you've got the right gasket?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!