I just finished the current playthrough and I am surprised the cops think they could get away with that. I am going off on the assumption that everyone has reached that part so far.
For starters, a police officer is not allowed to touch you unless they have proof you have actively committed a crime. So the detective harassing Gabby was in the wrong. Sure, Guy should not have done what he did, but that doesn't justify the overall assault by the police in turn, not by that proportion. Guy's lawyer will tear that station a new one, especially with a witness who can easily testify the detective touched Guy first without any suspicion at all that he was committing a crime.
The detective could get into trouble for that alone. On top of that, I could easily see he was not just talking to Gabby, but harassing her just because her dad had been arrested on alleged drug charges.