Pretty much all cars made by IMP in the 1970s were in some form or another mostly just upgrades of the cars built in the 1960s. All but one that is.
In 1979 IMP launched something that was completely different from anything else they made at the time, because the 1979 GSX was the first IMP vehicle ever to feature front-wheel drive. Unlike most front-wheel driven cars the GSX was not a fuel sipping econobox, but a powerful sports saloon with a screaming DOHC Flatplane V8 engine loosely based on the existing Crossplane G-Series V8. Clearly the car was aimed at the North American market. Flatplane V8 engines are completely unique to the GSX, no other IMP Automobile was ever equipped with such an engine, at least from the factory. Another “feature” of the GSX was its deliberately unassuming appearance.
The first iteration of the GSX developed 240hp from 3.6L of displacement and equipped with a catalytic converter by factory.
From the start IMP knew that the GSX had a very small profit margin, nevertheless it had an unusually long production run as IMPs substitute Halo car with several updates along the way. The first of those upgrades happened in 1982. That year, the engine was enlarged to 4.1L and power increased to 270hp. 1982 was also the year in which an automatic transmission was added to the options list. The design meanwhile stayed unchanged.
The first major update happened in 1985. That year marked a defining moment for the GSX nameplate, as the Automatic model was now treated as a seperate car from the Manual. The Automatic model was tuned for comfort and refinement, whereas the Manual model had a stronger focus on performance than before. The main mechanical change however was the introduction of a new 32V V8 with electronic fuel injection. The Automatic equipped model had a slightly larger 4.5L engine, while the Manual retained the 4.1L capacity. Power for both models was identical at 307hp, but the 4.5 had better torque at low rpms while the 4.1 was more responsive and generally performed better above 5500rpm. A common modification for these engines are a 4.5 crankshaft and conrods paired with 4.1 heads and intake, a combination good for ~340hp. Accompanying the new engine was a mild facelift.
The final update came in 1990. The GSX received its first major visual facelift with a completely redesigned front end for better aerodynamics (0.37cd vs 0.41cd for pre-1990 cars) and
a slightly enlargened luggage compartement. The engine also received a mild revision, the 4.5L GSX L now produced 336hp while the GSX S made 346hp from 4.2L, thus making it the most powerful front wheel drive car in the world by a long way until the 2010 Ford Focus RS500.
In this form, the GSX stayed in production until 1995, when the second generation car was released. The original GSX had the longest production run of any IMP passenger car, but it only sold 23,664 units over the 16-year period.