Synchrony and Comenity are not easily willing to honor GW requests, but your experience may be different. You will have to just hope your letter lands on a good lap.
If a creditor has sold a delinquent debt after taking a charge-off, they have received on pennies on the dollar amount of the debt.
While they have also recouped a bit of the loss via tax writeoff of the CO, they have taken a net loss after the sale of the debt, and dont receive another penny if the consumer later makes payment to the new owner.
Having taken a net loss on their account, they are usually not in a mood of grant of good will to a consumer that caused them a loss.
You might have some compelling reason explaining why you let the delinquent debt go to the extreme of having reached the point where they determined that it was unlikely to be paid, which is what a CO is, but they may not be too receptive too such pleas.