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.
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.
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.