For the final year, 1987, Buick introduced the GNX. Produced by McLaren Performance Technologies/ASC, Buick underrated the GNX at 276 hp (206 kW) and a very substantial 360 lb·ft (488 N·m) of torque. This was created to be the "Grand National to end all Grand Nationals." Changes made included a special Garrett turbocharger with a ceramic-impeller blowing through a more efficient intercooler and a "CERMATEL (Ceramic/Aluminum) coated" pipe connecting the intercooler to the engine. A GNX specific EEPROM, low-restriction exhaust with dual mufflers, reprogrammed Turbo Hydramatic 200-4R transmission with a custom torque converter and transmission cooler, and unique differential cover/panhard bar included more of the performance modifications. Exterior styling changes include vents located on each front fender, 16 inch black mesh style wheels with VR-speed rated tires, and deletion of the hood and fender emblems. The interior changes of the GNX included a serial number on the dash plaque and a revised instrument cluster providing analog Stewart-Warner gauges, including an analog turbo boost gauge. GNX #001 is the 1986 prototype currently owned by Buick and sometimes makes appearances at car shows around the US. The GNX had a ladder bar that ran from the mid-section of the car to the rear axle, so as to increase traction. This is also the reason why a GNX will actually lift the rear end up when the car is about to launch heavily.

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