During the closing stages of the first half, Chris Smalling tripped Dwight Gayle just inside of the area, but the official waved away the Magpies' appeals for a foul.
However, Gallagher has suggested that referee Craig Pawson may have been distracted by trying to decide where any infringement took place.
Gallagher told Sky Sports News: "The simple answer is yes. He didn't play the ball, and as I've said many times, it's all about the ball.
"I actually think that because where Craig is, I believe he may be assessing whether it happened inside the area or not, and because of that he loses sight of whether it is actually a foul. He completely forgets, I think."