Already on a yellow card, Kolasinac contested a high ball with Ezequiel Schelotto, only to make connect with the defender who had to be substituted due to the Bosnian's strong challenge.
Referee Stuart Attwell opted against handing Kolasinac a second booking, but Gallagher has revealed that he had expected the Gunners player to be sent off.
Gallagher told Sky Sports News: "Wrong decision. I was watching it live and expected him to get sent off. I thought there was enough force in that tackle to be considered reckless. It was certainly a second yellow card. I think in Stuart's view, he has seen them head the ball, and has seen them come together.
"It's easy when we dissect and look at it here, and in the speed of the match I think he may have thought he's headed the ball."
Brighton were still able to record a 2-1 victory at the Amex Stadium.