The England international would join Mason Mount, Lewis Hall, Ruben Loftus-Cheek, Ian Maatsen and Callum Hudson-Odoi as products of the Chelsea youth system to be sold over the past two years, while Armando Broja and Trevoh Chalobah are also expected to leave before the end of August.
Chelsea have needed to recoup funds after splashing well over £1 billion ($1.3 billion) in the past four transfer windows to revamp the squad under new ownership led by LA Dodgers co-owner Todd Boehly and private equity firm Clearlake Capital.
The sale of players who came through the academy is particularly helpful for clubs to comply with financial regulations as the full transfer fee is recorded as a profit in the books.
Profit and sustainability rules that limit Premier League clubs to losses of £105 million ($134 million) across a three-year assessment period have been blamed for a slow summer transfer window so far.
Last season Everton and Nottingham Forest were docked points for breaching the regulations, while newly-promoted Leicester could face one this season for breaches in the 2022/23 campaign.
"This is not Chelsea's problem, these are the rules. All the clubs at this moment are compelled to sell players from the academy because of the rules. It's all of the Premier League clubs' problems," said Maresca, who joined Chelsea from Leicester in June.
"It's a shame because in Italy, we have (Francesco) Totti with Roma, 20 years with the same club - one club man. We love that in football, the fans want to see that. But with the rules now it is different from the past."
On whether the rules need to be changed, Maresca added: "If they want to protect academy players, then probably yes."
Gallagher was Chelsea's captain for much of last season due to Reece James' injury troubles.
Despite fan unrest at his potential departure, the 24-year-old Gallagher appears headed for Spain to join Atletico in a £33 million deal.
Gallagher had just one year left on his contract at Stamford Bridge and reportedly rejected the offer of a two-year extension.
"In this moment, there is something between the club and the player - an agreement to stay or to leave," said Maresca.
"These kinds of things happen in every club, every summer and in the end the final decision will be good for everyone.
"Hopefully the final decision will be good for Conor and the club."
