Looking at the GT2 Configurator, which is in English under 'Other Countries' in 'Europe' on the Porsche.com website it appears there are three choices for interior:

Black - full leather ie no alcantara
Black- leather/alcantara
Grey - leather/alcantara

Personally I think the alcantara wheel/gearstick/handbrake etc makes the car feel far more like a GT3/RS in nature, whereas the full leather makes it feel more like a Turbo.....