The Hammers’ 100 per cent start to the league campaign came to an end.
The Eagles remain winless under new boss Patrick Vieira but this was a step in the right direction as they twice came from behind to earn a point at the London Stadium.
West Ham went into the game in fine form as manager David Moyes named an unchanged side for the third game in a row, seemingly justified in his selection when both Pablo Fornals and Michail Antonio had put the hosts ahead.
Another fine individual display for Antonio saw him assist and score but this time it was not enough to seal all three points as West Ham dropped points for the first time this campaign.
Vieira’s side had not scored in two league games and a Carabao Cup tie but Chelsea loanee Gallagher ended the barren spell with two smart finishes.
Palace had Joel Ward to thank for preventing an early own goal as Joachim Andersen’s attempted clearance went goalwards, with Ward sliding in to clear off the line.
Antonio would be heavily involved in the opening goal, initially latching on to a through ball from Said Benrahma. He checked back and fed in Fornals who exchanged passes with Antonio before slotting home, with the Palace defence bamboozled.
While the goal was the only thing separating the sides at the interval, the gulf in class in the opening 45 minutes was huge – something Palace looked set to change after the break.
The visitors had seen enough of the ball in the opening 45 minutes but had failed to test Lukasz Fabianski in the Hammers goal.
Wilfried Zaha was cutting an increasingly agitated figure and that was not helped when he blazed over at the start of the second half.
The Ivory Coast forward would play a huge hand in the equaliser, however, crossing for Christian Benteke to nod down into Gallagher whose close-range effort had just enough on it to beat Fabianski.
Despite now being on the back-foot, Antonio proved what a fine striker he has become, out-muscling Andersen and thrashing past Guaita to put West Ham in front.
This time the lead would last just two minutes as Palace skipper James McArthur found Gallagher inside the box and his drag back and finish drew Vieira’s men level once again.
Both sides set about finding a wining goal and, despite West Ham having more of the ball, it was Palace who came closest as Marc Guehi headed a Gallagher free-kick over the top in stoppage time.