Existing Xbox Gold account.

The best advice is to contact Microsoft support in this case.
Link

It does sound like you have a duplicate account.

The first thing you should do is try going through the account recovery ("forgot password") process for your duplicate Microsoft account (the one with the Gmail address).

If this succeeds, you can then add a new email address (perhaps a new Outlook.com mailbox) to that account and then delete your Gmail address from it. This will free up your Gmail address so you can add it to your primary Microsoft account (the one with
the Freeserve address). Then set your Gmail address as the primary address and remove the obsolete Freeserve address.

When that's all done, you can delete your duplicate account through the account security page (https://account.microsoft.com/security/) under "more security options".

Also for a little more info on the prize. Nowhere does it stipulate that you cannot use the One Month Code with an existing Microsoft Account Which already has Gold. When looking at the xbox.com/usagerules , No where does it state that a "Xbox LIVE Prepaid
1 month Gold Membership Card." cannot be added to an existing gold account.

Gold Family – 12 Month $99.99 ($8.33 per month)

As an existing Gold member, if you upgrade to Gold Family Pack, your existing balance will be applied to your new account.

This is the only account on my Xbox that was able to claim battlefield one off of games with gold. Non of the other accounts existed during that month