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.