GT1, also known as Group GT1, was a set of regulations maintained by FIA. The class was created in 1994 as the top class in BPR Global GT. FIA took over BPR in 1997 after which the series was renamed FIA GT Championship. GT1 was dissolved in 2011.
Some of our favorite and most legendary race cars competed in GT1 - some were built specifically for it. Automobilista 2 focuses on the late 90s prototype era where cars were homologation specials. For example the Mercedes-Benz CLK GTR only shared instrumentation, front grille and four headlamps with the regular CLK208 model.
These homologation specials were so extreme that McLaren wasn't realistically able to compete with them after 1998. They had to build three special models of the F1 called the F1 GT just to homologate an upgraded body. But it wasn't enough in the long run against specially built prototypes.
Compared to GT3, GT1 cars are more powerful and lightweight. They also don't have traction control or ABS so you have to be extra careful to avoid throttle oversteer and spinning. It's easy to lock up under braking, run wide and get flat spots.