If you bought Gold on either your Xbox or PC (using a debit/credit card), automatic renewal is on by default. The only way to turn it off is to call Xbox Support, which can be reached at 1-800-469-9269. Tell them you want the feature off. It took me about a half-hour, most of that time I was on hold... Shitty service.
I know there used to be a way to check if auto renewal was on or not on the Xbox website, but it looks like they got rid of that ability with the new site revamp. Not like you could turn it off anyway. Again, shitty service.