They keep changing class designations and classifications, but I think the American Lemans Series uses the Classes GT1, GT2, GT3, and GTS
When you have a car like a Porsche GT2 (rwd twin turbo in 2001) it is factory ready to race in a GT2 race (safety equipment, hp requirements, rwd, etc.)
GTS used to be GTQ which meant that if your car could run fast enough and met saftey specs, and be competitive, you could run it.
Like I said, They keep changing things, and I have been sans speedvision for too long.