Pepe has been in outstanding form for Lille this season, scoring 22 league goals from 37 appearances as Galtier's side secured a second-place finish.
His performances have reportedly drawn interest from a number of high-profile suitors, and Galtier conceded Lille has little choice but to cash in on the 23 year-old.
"Nicolas Pepe will leave. Life's like that, football's like that today," Galtier told Canal+. "He is a player of the highest level who will have a big decision to make on the choice of his next club.
"There will be a battle between the great European teams. I hope that he will take good amount of time to think about it, do his research."
Pepe scored twice in Lille's 5-0 win over his former club Angers.
Premier League clubs Liverpool and Manchester United have been credited with an interest, though Paris Saint-Germain is also reportedly eager to sign the Ivory Coast international.