Nasri recently signed a new deal with Marseille, which are seeking at least £12 million for the midfielder.
"Before thinking about bringing in reinforcements over the summer we need to know who is going to stay, and above all the players I want to keep," Gerets told L'Equipe.
"If there is a big offer for Samir, then of course he will leave.
"I can't criticize the club because it is the same everywhere. When there is a huge offer made, players leave."