Chants of “Chelsea rent boy” were heard after Gallagher was booked for a foul on Raphinha in the first half. Raphinha would go on to score the winner for Leeds as he converted from the penalty spot in stoppage time.
The game was part of the Premier League's Rainbow Laces campaign, with the rainbow colours on display in many parts of the stadium.
A Leeds United spokesperson said: “Overall the support from Leeds United supporters at last night’s game with Crystal Palace was magnificent. Once again the fans got behind the team and played a huge part in pushing the players on to get the late winner.
“However, we are aware of a homophobic chant directed at a Crystal Palace player and we want to be very clear that this is unacceptable.
“We will continue to work closely with Marching Out Together and all of our supporter groups to ensure that we do everything within our power to eradicate homophobia and any other form of discrimination from our football club.”
A spokesperson for Marching Out Together continued:: “Last night was our Rainbow Laces game. The club have actively supported the campaign again this year - with players discussing LGBT+ inclusion on podcasts and addressing schools, they have been photographed holding rainbow flags and they wore special pre-match shirts.
"The board and coaching staff all wore Marching Out Together badges, and there was active promotion of Rainbow Laces on the club’s social media site - which went well beyond any requirements from the Premier League.
"The powerful support offered by the club to Marching Out Together’s work lasts throughout the year; with the culture of support led from Angus Kinnear, our CEO, and supported by his senior management team.
"The work we do with the club receives the support from the overwhelming majority of Leeds fans, who welcome inclusion and support for the LGBT+ community.
"However, work still needs to be done, and unacceptable chanting can still occasionally be heard. The rent boy chant last night is one example of that. It is unarguably homophobic and unacceptable at all times. We wrote a piece condemning the use of the rent boy chant and explaining why it is homophobic last year.
"It was certainly regrettable to hear last night - but the club’s ongoing powerful message of support for equality resonated much stronger on a night where we could celebrate that things are changing for the better in the game of football, when it comes to LGBT+ inclusion.”