John Wesley on 2 Samuel 15:18

Gittites - Or rather strangers, as Ittai their head is called, ver.19, and they are called his brethren, ver.20. 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, were won to embrace the true religion, and had given good proof of their military skill, and valour, and fidelity to the king.