best advice I can give you is enter the game you purchased the gold from, and at the bottom right select the gear icon. then select bug report, file a report for the game developers to investigate from, make sure to choose "payment issue", and list all pertinent information to assist them in the investigation.
and of course remember to try and be civil and polite, as they are there to help you.
Being as this should be a private matter between you and the person investigating this, I would appreciate this being the end of the conversation, as to protect not only there right to privacy, but yours as well. 
