The G5 has a 17 tooth sprocket and 19" wheels.
The C5 has an 18 tooth sprocket and 18" wheels.
It is my understanding that the G5 can take the 18T sprocket, but that's as far as it can go.
I believe that the hubs themselves are the same. But the wheel diameter is different.
If you used 19" wheels, with tires that aren't "squattier" in profile, this should result in a longer roll-out distance, and make the effective gearing taller, by maybe the same as what a tooth on the sprocket would give you(if you're lucky). I don't know the exact ratio change, because I don't know the roll-out of whatever tire would be selected.
However, the theory would be that a longer roll-out of the larger wheel and tire would make the gearing taller by some amount.