The Philistines gathered to sacrifice to which god when they mocked the blind Samson?

Answer: Dagon

They praised Dagon for delivering Samson into their hands and called him out of prison to entertain them.

“Now the lords of the Philistines gathered together to offer a great sacrifice to their god Dagon. They rejoiced and said, “Our god has delivered Samson our enemy into our hands.””— Judges 16:23 (BSB)