Albert Barnes Commentary 1 Samuel 23:2-4

"Therefore David inquired of Jehovah, saying, Shall I go and smite these Philistines? And Jehovah said unto David, Go, and smite the Philistines, and save Keilah. And David`s men said unto him, Behold, we are afraid here in Judah: how much more then if we go to Keilah against the armies of the Philistines? Then David inquired of Jehovah yet again. And Jehovah answered him, and said, Arise, go down to Keilah; for I will deliver the Philistines into thy hand." — 1 Samuel 23:2-4 (ASV)

If Gad was with David at the forest of Hareth (1 Samuel 22:5), where he inquired of the Lord for him (1 Samuel 23:2, 4), but did not accompany him to Keilah—and if Abiathar’s flight occurred at the very time David was in Keilah—we have another striking instance of God’s watchful, providential care. God sent Abiathar to supply Gad’s place at that critical moment.