Fuming Birmingham City boss Steve Cotterill hits out after red card in Sunderland draw
The striker, who had earlier given the Blues a 16th minute lead, was given his marching orders in the 67th minute by referee Oliver Langford.
He was booked for a dive before picking up a second yellow for a late foul on Sunderland striker James Vaughan.
The Black Cats, who had drawn level through Lewis Grabban's 12th goal of the season, failed to make the most of their man advantage late on.
The decision to send Gallagher off angered former Sunderland assistant Cotterill.
"There was three points we deserved," said Cotterill.
“If we start with it, I think it was a foul on Cheikh Ndoye for Sunderland’s goal.
"Sam Gallagher’s two bookings are the softest bookings you will ever see in your life.
"You can understand if it’s one of them, but to go and hit him with the other one. Sam Gallagher is 100 per cent an honest boy. He hasn’t dived.
"When he gets that touch, if he’s steamed into the goalkeeper what would happen then? So he has to go round him.
"For me, that’s really, really harsh. If that’s at St Andrew’s he doesn’t get booked for that.
"And the same for the second one - there’s no way that that’s a foul, no way.
"If it’s not a foul on Cheikh Ndoye, for Sunderland’s goal, it’s not a yellow card for Sam Gallagher.
"And that tipped the balance of the game for me."
The draw saw Sunderland drop back into the Championship relegation zone ahead of the Boxing Day trip to Sheffield United.