I have gone through your problem.
You have undergone laparoscopic surgery
for gall bladder
removal. During this surgery carbon dioxide gas will be sent into your Peritoneal cavity
(abdominal cavity). After the surgery some patients may experience pain under the ribcage or pain in the shoulder. This usually subsides in one or two days. As your nurse said this is due to the gas left in the stomach. Don't worry about this pain.
Good luck and wish you a fast recovery.