Chelsea recorded just their second Premier League home win of the season against Brighton, despite being reduced to 10 men before half-time.
Chelsea were cruising midway through first half after goals from Enzo Fernandez and Levi Colwill.
Facundo Buonanotte halved the deficit for the visitors shortly before half-time but there was still time for Gallagher to pick up a second yellow card, changing the complexion of the match.
Chelsea were awarded a penalty when James Milner was adjudged to have fouled Mykhailo Mudryk and Fernandez made no mistake to restore Chelsea's two-goal cushion.
Joao Pedro pulled a goal back in the 92nd minute to ramp up the pressure on Chelsea and the home fans' hearts were in their mouths when Brighton were awarded a penalty for a handball by Colwill in the 101st minute.
But the referee reversed the on-field decision after reviewing the incident on the pitchside monitor and Chelsea held on.
Ollie Watkins headed home a cross from Moussa Diaby in the 90th minute to rescue a point for Aston Villa, who remain in fourth place. “It's a good point. We were aware it would be difficult to get a victory today,” said Aston Villa manager Unai Emery.
“We struggled in moments and we were resilient. We tried to get on the ball and score and we did that. This point is not the best but it is not the worst.”
Bournemouth's Antoine Semenyo opened the scoring early before a fine individual goal from Leon Bailey.
Dominic Solanke put the home side back ahead early in the second half and Bournemouth looked like hanging on for a win before Watkins' late equaliser.
“We deserved to win the game but against this type of opposition this can happen. One point is not enough for us today,” said Bournemouth manager Andoni Iraola.
“Both sides are good in transition so every time we lost the ball it was dangerous, their goalkeeper was excellent and that's the reason why we didn't win.
“The point is good for us when you are losing a match. We will try to use this point. We prefer to control the game but it was not easy, they had very good chances and our goalkeeper was fantastic.”
At the London Stadium, Mohammed Kudus put West Ham ahead in the first half, before Odsonne Edouard equalised in the second period to earn a point for Crystal Palace.
“We had to survive a period in the last five minutes of the game and added time when they were getting the ball into the box,” said Eagles manger Roy Hodgson
“It would have been very harsh if we had not got something we got from the game. You don't want to lose three games in a row. I am pleased the way the players rallied around.”
