Without the specifications on the BMW carbon ceramic system it is difficult to say how it stacks up to the G-Power offering. What we do know is the G-Power brake kit is 420x40mm with 6-piston calipers. Those specs definitely make for one heck of a brake setup.
Now the question really will be what will be cheaper, the G-Power carbon ceramic brake it or the factory BMW carbon ceramic brake option? We do not know as G-power has not released pricing and BMW has not priced their option. Additionally, it is easier to check the option box rather than retrofit these as one can not simply include aftermarket parts in the purchase price. This means most people who have carbon ceramic brakes on their M5 or M6 will have the factory option as it can be financed.
Still, for the M5 owner who absolutely has to have carbon ceramic brakes G-Power has you covered. Just bring cash, lot's of it.