Matthew Poole and continuators on 2 Samuel 15:18

18 And all his servants passed on beside him: and all the Cherethites, and all the Pelethites, and all the Gittites, six hundred men, which came after him from Gath, passed on before the king.

Of whom see the notes on chap. 8. 18.

who were either, first, Israelites by Birth, called Gittites, because they went with him to Gath, and abode with him in that Countrey. Or rather, Secondly, Strangers, as Ittai their head is called, v. 19. and they are called his Brethren, v. 20. and probably they were Philistines by Birth, born in the City or Territory of Gath, as the following words imply, who by David's counsel, and example, and the success of his Arms, were won to embrace and profess the true Religion, and had given good proof of their Military skill, and valour, and fidelity to the King.