Gallagher scored five Premier League goals last season and registered seven assists, featuring in 37 out of 38 league games under former boss Mauricio Pochettino.
Despite Chelsea ending the season strongly with five wins in a row to earn a sixth-placed finish, Pochettino departed and has been replaced by Enzo Maresca, who helped Leicester City win promotion back to the top flight.
All-action midfielder Gallagher said he was “sad” to see Argentine Pochettino depart but “excited” to work under Maresca, a former assistant to Pep Guardiola at Manchester City.
“I’ve come a long way this season,” Gallagher said.
“Pochettino has done so much for me. I was sad, sad for him and the coaching staff.
“Credit to Pochettino for what he has done for me. He has given me a lot of responsibility and trust at Chelsea and it puts me in a better position to be a bigger part of the squad for this tournament.”
England start their Euro 2024 campaign on Sunday against Serbia before they face Denmark on 20 June and Slovenia on 25 June in their other Group C games.
