Replays showed the official looking away from the free kick, and Gallagher has acknowledged that lessons must be learned after the incident.
Gallagher told Sky Sports News: "I think it raises an interesting point. I think the referees will have to consider where they are going to stand at this type of free kick. He has gone for what he thinks is priority, with the priority being where the ball is going to drop.
"The learning point is that they will need to think of a different angle to cover both options or use a different official."
Newcastle went on to suffer a 3-2 defeat at Old Trafford.