Affirm, I am pretty sure that Garrett is professional public relations guy. If I am right there is an episode in which is mentioned that Garrett once worked for the opposition of Mayor Russo (first Blue Bloods Mayor) and wrote some ugly text about Frank.
//Edit: I looked it up, it is episode 3x16 "Quid Pro Quo", see https://en.wikipedia.org/wiki/Blue_Bloods_(season_3)#ep60