Albert Barnes Commentary 1 Chronicles 21:18

"Then the angel of Jehovah commanded Gad to say to David, that David should go up, and rear an altar unto Jehovah in the threshing-floor of Ornan the Jebusite." — 1 Chronicles 21:18 (ASV)

It has been observed that angels are presented as intermediaries between God and the prophets only in later books. This is undoubtedly true, and it is certainly unlikely that the sources from which the author of Chronicles drew mentioned Gad receiving his knowledge of God’s will from an angel. This detail may be regarded as coming from the writer of Chronicles himself, who expressed the event recorded by his sources in the language of his own day (see Zechariah 1:9, 14, 19; 2:3; 4:1; 5:5; and elsewhere).

However, this language should not be regarded as merely rhetorical, but as strictly in accordance with truth. For angels were undoubtedly employed as intermediaries between God and the prophet as much in the time of David as in the time of Zechariah.