How do you play a G key with a C accordion?

Just pull, man.

You are playing cross key, as you would with a blues harmonica.

You just centre on the 5th button down (G) rather than the 3rd button (C)

All the notes for G are the same as the C scale apart from the F instead the F sharp, which as F is just the dominant 7th instead of the major isn't a big deal.

In the end though, you probably don't really need to know, as when you are working a tune out your fingers will just find the right place.

As Nick said just pull.