If your flight is BA operated, paying for checked baggage on a basic fare doesn't change the fundamentals of the ticket, you just get the extra paid for bag. To summarise a long running discussion point, your Gold card - at BA locations - will almost certainly allow you to take that bag to 32 kgs rather than 23 kgs, but that's as far as it goes. The careful wording in the previous sentence is due to the lack of documentation on the subject, and the fact that those who ask about it rarely return to share the outcome.