Gallagher spent last season on loan at Crystal Palace in the Premier League and massively impressed.
The 22-year-old scored eight goals across 34 games in the Premier League and was named Crystal Palace Player of the Year for 2021-22, working under boss Patrick Vieira.
He was again linked with a loan move for the 2022-23 season, with Crystal Palace interested in taking him back, while other Premier League clubs were also keen.
Aston Villa, West Ham, Everton and Newcastle United were all linked with loan moves for Gallagher.
“I don’t think in terms of leaving the club, but another season on loan for Gallagher would have been great for him,” Johnson told SafeBettingSites.
“Personally I would have liked him to carry on his development, he was clearly enjoying himself.
“I’d rather have Chelsea have let him go back out on loan and then re-evaluate the situation at the end of the season.”