What was Gideon doing when the angel of the LORD appeared to him?

Answer: Threshing wheat at a winepress

Gideon was threshing wheat in a winepress to hide it from the Midianites when the angel called him a mighty man of valor.

“Then the angel of the LORD came and sat down under the oak in Ophrah that belonged to Joash the Abiezrite, where his son Gideon was threshing wheat in a winepress to hide it from the Midianites. And the angel of the LORD appeared to Gideon and said, “The LORD is with you, O mighty man of valor.””— Judges 6:11-12 (BSB)