Asus seems undecided on whether they're going to allow official G-SYNC support on the existing G751's purchased before now (? - I don't know the official date, they just said this month I believe).
Please "sign" this petition by posting... even if you don't think you'll need the feature, you'll want it someday, trust me.
ASUS: Even if there are licensing fees involved or whatever, if that is indeed the reason you wouldn't offer it to existing owners.. then at least let us pay the fee for it! Something... anything!
The amount of brand-loyalty to be gained from giving this feature to existing owners would build more goodwill than you're imagining. Market it! Tell the world that you were the first to give this major support release to existing owners!