Sorry if I'm asking stupid question, no one has to explain but is there some link or website with good explanation which are exactly differences for the cars in each class.. For example in in Super GT I kind of understand GT300 and 500 classes but in other organizations for me who is kind of new to all this is very confusing.. Thanks in advance
If you go to Wikipedia, there's a lot of information about the different classes. Here's how i understand the GT classes;
GT3 (IMSA: GTD): originally created by SRO, production based cars, around 500-550 HP rear-wheel drive cars, they have some aero and gizmos (ABS, TC. You can see this category basically everywhere, SRO series, IMSA, some ACO-backed series, 24H Series, Super Taikyu and partially in Super GT's GT300 class. There are Pro, Pro/Am and Am entries depending on the series and there are a lot of manufacturers producing GT3s.
(LM)GTE (IMSA: GTLM): ACO's GT category, production based but more flexibility compared to GT3, e.g. the Porsche 911 GT3 R (GT3 car) has a rear engine like the production car while the 911 RSR (GTE) is mid-engined so that they can put in some additional aero. It's the manufacturers' playground in IMSA (so far, no idea how this will change in 2020 after Ford officially stop their direct engagement) but WEC has Am GTE cars, too. Significantly less cars available. The Ferrari 488 and the Aston Martin Vantage deserve extra mention as they can be converted from GT3 to GTE and vica-versa. In general, GTEs are faster than GT3s and don't have ABS (from what I gathered during this year's Rolex 24h), not sure about TC
GT4 (IMSA: GS): as far as I know, also an SRO-created category, also production-based but less changes allowed compared to GT3 and more Am focused, although you can find a lot of pros there. It also offers worldwide a lot of car variety although not all cars race everywhere. For example, I've seen the Panoz only in the US while the Alpine A110 only in France but I maybe mistaken.
GT2 (the new SRO one): again, SRO-created, Am focused, production based cars, seemingly similar concept to GT4 but with stronger cars, for instance Porsche runs the Cayman in GT4 and will run the 911 GT2 RS Clubsport in GT2.
GTC: GT cup cars like 911 GT3 Cup or Lambo Super Trofeo. These are the spec cars from the respective series, you can see them racing in some events or series outside of the one-make series. Examples would be 24H, Bathurst 12H or British GT (and most likely more but I'm too lazy to look up more for now
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Some series like the 24H, Trans Am and Super Taikyu have their categories named differently, let us know if you have specific questions!
EDIT:
completely forgot about GT500... they look GT but they're more spec prototypes than GTs, just like DTM. 2L I4 turbocharged engines, lot of aero. Not sure about GT500 but DTM has carbon monocoque and both series sport bodies that resemble the road cars somewhat. Big difference between GT500 and DTM is that DTM has spec tires (Hankook), GT500 has tire war (4 manufacturers if I'm not mistaken).