Found a link on the Lemans site for those interested. Basically comes down to BHP and Weight:
GT1 cars with a power output that can reach 560bhp with a minimum weight of 1,200kg or 600 bhp with a minimum weight of 1,250kg
GT2 cars are 450-500 BHP min weight 1145kg
http://www.lemans.org/en/races/24h/discover-endurance/gt1-category.html
http://www.lemans.org/en/races/24h/discover-endurance/gt2-category.html
GT1 cars with a power output that can reach 560bhp with a minimum weight of 1,200kg or 600 bhp with a minimum weight of 1,250kg
GT2 cars are 450-500 BHP min weight 1145kg
http://www.lemans.org/en/races/24h/discover-endurance/gt1-category.html
http://www.lemans.org/en/races/24h/discover-endurance/gt2-category.html