It's unclear and seems very much to depend on which way the wind is blowing when you make your booking. By looking at the terms, it appears to forbid any non-BA carriers. Point 7 of BA's GUF terms states -
7. Gold Upgrade vouchers cannot be applied to a booking containing any carriers other than British Airways.
However, there is evidence on there that it hasn't been allowed. I guess all you can do is try...