There is a link in this story with full side by side comparison of the G56 and Cup car specs.
Headlines
Weight - 1342kg (about 245kg less than Cup spec!)
Gearbox - 5 speed paddle shift (vs 5 speed sequential shift)
Wheels/tyres - slightly wider wheels and wider rear tyres for G56
Fuel cell - 127 litres (about 48 litres more than Cup)
https://motorsports.nbcsports.com/20...mans-next-gen/