I think there were only three vintages of the Gran Reserva available: 1979, 1980, and 1981.
They are no longer being produced or released by Macallan, and have subsequently become collectors' items. Their flavour profile is enormously sherried, if not a touch woody. I would not call them particularly "representative" of a typical 18yo Macallan.
The 25yo however is still being regularly bottled and released, and although recent bottlings appear to have been scored a little lower than previous bottlings, it is still a wonderful malt, with a beautiful flavour & complexity, and handsomely packaged.
The 25yo and the Gran Reserva are similarly priced, but if the recipient of your gift is likely to drink the whisky rather than stare at the unopened bottle on his mantlepiece, I would suggest the 25yo.
(P.S...If this reply has been at all helpful to you, I'll happily accept whichever bottle you elected not to give as the present!