Unfortunately, you cannot ask your DD to change the baby's name. You made the decision to not tell your children about the affair, and you can't make that request without first telling your daughter and son-in law about the affair. It's not fair to them. Especially if they had a difficult time selecting a name.
You’ll call your beautiful granddaughter by a nickname that has nothing to do with the sound of her official name. Take a breath. I think you’ve had a lot to handle but this part I think will be totally ok.
Like this PP, I think you come up with a special nickname for your DGD that you call her by and you just learn to accept the problem.
What would you have done had your granddaughter been born and named before your husband had the affair? You would have had to live with that painful coincidence as well.