A GXP BB is 24 mm on the drive side and 21 mm on the non-drive side. Some aftermarket BBs use a 24 mm Shimano type BB with a reducer sleeve in the non-drive side.
Likely the crank was installed without grease on the shaft or it's been in there a long time. That sleeve will need to be removed and then maybe a new BB. If you get the sleeve off in one piece then use lots of grease to reassemble.