- The GTX was an extra package available in the 1967 Plymouth Belvedere. This performance package included a 375-horsepower, 440-cubic-inch engine. The Belvedere was a stylish vehicle, and the high-performance GTX was available as a two-door sedan or convertible.
- Plymouth released the GTX as its own high-performance model in 1968. The headlights, tail lights and grille were altered to be more aerodynamic, and the GTX had a new hood design. The tires were wider and the suspension was modified to improve the car's handling. Plymouth also added many performance extras as standard equipment.
- The 440-cubic-inch engine remained standard throughout the life of the GTX. A three-speed automatic transmission was standard, but a four-speed manual floor shifter was also available. Carburetor adjustments allowed the 1969 version of this engine to produce 390 horsepower.
- The GTX was phased out in favor of the better-selling Plymouth Road Runner. The final year for the GTX and its 440-cubic-inch engine was 1971. The GTX package was offered as a trim option for the Road Runner in 1972.