The JGTC started out as being quite "touring car" in nature. But as the years have gone by, they have become more prototype and LeMans in nature. For '03, I think they are now allowing tube frame carbon fibre chassis I think.
They have a good article explaining the changes for '03.
http://www.jgtc.net/race/inside/2003e/repo_e031.htm
It's a shame I think. They are losing their showroom identity.