GT stands for Gran Turismo or Grand Tourisme, that is, Great Tourism or Large Tourism. They are cars for endurance races. They originated in cars that could be used for long distances.
GT1 is the most powerful series, with cars going around 650 hp. GT2 are cars with something like 450 hp and closer to production cars. GT3 are even more limited, I've not heard about the specs, but the drivers must be non-professional, like in the new GT4. I quote: "The basic idea, which has encouraged SRO Motorsport Group to create the GT4 European Cup, is to extend the proven GT3 concept to a less expensive second-tier GT category of vehicles".
The cars in GT1 are Aston Martin DBR9, Corvette C5 and C6, Lamborghini Murciélago and Maserati MC 12. In GT2, "the new Porsche 997 GT3 RSR and the Ferrari 430 GT2 are the main contenders".
NGT, I think, is the Nippon version. There are several national cups in the GT class, that are not homologated in the same way. There is the British GT and the ADAC GT Championship for "oldies".
As you probably know, GT1 is a "balanced series". This means that:
"Cars which finish in the top three in either the GT1 or GT2 classes are awarded penalty weight. In GT1, they get 40kg for a first-place finish, 30 kg for second and 20 kg for third. In GT2, these weights are halved, so they get 20 kg, 15 kg and 10 kg respectively. Cars which finish from 4th and under progressively lose any penalty weight they might have accumulated. A 4th place finish loses 20 kg in GT1, with a loss of 30 kg in fifth place, and 40 kg are removed for sixth place onwards. The maximum weight that can be carried is 100 kg in GT1, or 50 kg in GT2. If a car carrying the maximum wins a race, they are given a 'supermaximum' of 150 kg in GT1 or 75 kg in GT2.
Penalty weight is also awarded for cars which may miss a race, or for race-by race entries."
The name comes from the "Grand Tour" that some britons made in the XVIIth century around Europe, starting in Bristol and ending in Paris to "undertake lessons in French, dancing, fencing and riding". Nothing very different from what Mr. Mosley has done in Paris these days, specially the fencing lessons...
From Paris you went to Italy and maybe Greece, and in the return leg visited Vienna and Berlin, with perhaps a stop in Flanders.
