Both the GT2 and the GT3 fall under the 911 bracket in Porsche's model line up but are very much established as models in their own right. The 911 GT2 is based on the 911 Turbo with the key difference being that the GT2 is rear wheel drive rather than 4-wheel drive while it's twin turbocharged engine differentiate it from the naturally aspirated GT3.
The GT2 first went for sale In 1994 during the 993 series 911s but it was a very rare car as it was created solely to be able to enter a GT2 racing car into Motorsport with homologation rules stating it must be production-based. The GT2 was then reproduced more successfully for the 996 and 997 series where it was actually intended to be a road car since Porsche had started focusing their racing cars on the GT3 instead.
Whether or not we will see a 991 series GT2 remains to be seen, it is a niche car which sits between the GT3 and the 911 Turbo and it's a fantastic performance focused car. Using twin-turbocharged engines like the 911 Turbo the GT2 finds it's own performance orientation by having a lighter chassis, upgraded brakes and stiffer suspension over the 911 Turbo.