Cyriel Dessers was penalised for a foul on Celtic defender Gustaf Lagerbielke following a VAR review.
Referee Don Robertson initially played on but he awarded a free-kick to Celtic after being called to his monitor by video assistant referee Alan Muir.
The decision denied Rangers the opening goal and they went on to lose to Kyogo Furuhashi’s strike in first-half stoppage-time.
Gallagher told Sky Sports: “I think the referee has got it absolutely correct on the pitch. I don’t think it’s a foul, I think Dessers gets in front of him. He gets in front of him and he has committed no foul at that point.
“All I can think is when he looks the screen is he thinks he pushes him. But if VAR hadn’t intervened, no-one would have complained. It’s certainly not a foul.”
However, former FIFA and Scottish top-flight referee Bobby Madden earlier agreed with the match officials.
Madden, who retired from refereeing in the summer following one season in the English leagues, wrote on Instagram: “The defender is in control. The defender’s action is to play the ball with his left foot. This is the important part.
“The attacker has not played the ball at this point and puts his foot in between defender and ball.
“It’s a foul anywhere on the pitch, no matter what happens thereafter.
“Had the attacker played the ball before the contact, fine, but he was never in control or in possession.
“Live, like most people watching, I would never think foul. Naturally you would think ‘he has p****d about and been caught on the ball’.
“But when you have VAR and all the angles you can’t allow a goal when you have that evidence.”