1 Chronicles 21:18 - Albert Barnes' Notes on the Bible

It has been observed that it is only in books of a late period that Angels are brought forward as intermediaries between God and the prophets. This, no doubt, is true; and it is certainly unlikely that the records, from which the author of Chronicles drew, spoke of Gad as receiving his knowledge of God’s will from an angel. The touch may be regarded as coming from the writer of Chronicles himself, who expresses the fact related by his authorities in the language of his own day (see Zechariah 1:9, Zechariah 1:14, Zechariah 1:19; Zechariah 2:3; Zechariah 4:1; Zechariah 5:5; etc.); language, however, which we are not to regard as rhetorical, but as strictly in accordance with truth, since Angels were doubtless employed as media between God and the prophet as much in the time of David as in that of Zechariah.