John Gill Commentary 2 Samuel 15:19

"Then said the king to Ittai the Gittite, Wherefore goest thou also with us? return, and abide with the king: for thou art a foreigner, and also an exile; [return] to thine own place." — 2 Samuel 15:19 (ASV)

Then said the king to Ittai the Gittite
Who was over the band of Gittites, the six hundred men, (2 Samuel 15:22) ;

why do you also go with us ?
one should think the king should not have discouraged any from joining and following him, when his numbers were not very large, and the in such fear on account of Absalom:

return to this place ;
to Jerusalem, where his station was:

and abide with the king ;
with Absalom, who set himself up for king, and whom the people perhaps had proclaimed as such in Hebron, where the conspiracy began:

for you [are] a stranger, and also an exile ;
not a native of Israel, but of another nation, and at a distance from it, and therefore not altogether under the same obligations to attend David in his troubles as others were; and by this it seems that he was a Gittite by nation, whatever the six hundred men were, and rather favours the first sense given of them in (2 Samuel 15:18) .