Okay sounds like you've just been scamed . I don't know what GS told you this but.
In event that your AP scam on either side of trade(giving or getting gold) you can't per se report that person for it. Selling/Buying AP codes is just a liability on yourself, that's why you should only sell/buy from trusted sellers/buyers. If you aren;t sure, don't take the risk. If others never heard of them/not sure, don't take risk.
If you even attempt a report against them you need evidence (screenshots, videos).also 800g is barely of any AP worth. Sorry about lost of your gold but you live and you learn mate