Where did David hide after leaving Gath, as about 400 men gathered to him?

Answer: The cave of Adullam

All who were distressed, in debt, or discontented rallied around David at the cave of Adullam, and he became their leader.

“So David left Gath and took refuge in the cave of Adullam. When his brothers and the rest of his father’s household heard about it, they went down to him there. And all who were distressed or indebted or discontented rallied around him, and he became their leader. About four hundred men were with him.”— 1 Samuel 22:1-2 (BSB)