If y'all are talking about the rear hatch glass, the gasket can absolutely be re-used (assuming it's in decent shape and not too crusty). I've had great success on several occasions removing the rear hatch glass with gasket and trim intact, setting aside while painting/repairing the hatch, then reinstalling the glass and gasket.
It is true that the front windshield gasket cannot normally be reused, since it is typically cut away during windshield removal to reduce the odds of breaking the windshield.