I have a DW8000, but don't play it much as I'm more of a vintage collector of synths instead of a player/programmer. So I'm not too familiar with all of its features. But I can, however, suggest that if the keyboard is in tune with itself (meaning you can play the chromatic scale and play songs and everything's in tune), but it's out of tune to A=440, then the only thing I can suggest is maybe the transposing on it may be programmed to transpose it up to the key it's in (C = G as you say). I honestly don't know, but figure I can at least suggest working with the transpose feature on it. Hope that helps.
Terry