The correct answer is Eddy Duran.
Gilda's body is dressed for summer. But if she'd been killed in February, she would be wearing winter clothing. That means Gilda was recently killed and stuffed into a hidden section of the freezer in order to trick the police into thinking she'd been there for some time. Since it's been four months since Gilda vanished, the note in her pocket must have been meant for someone other than Tony: the man who is wearing a tie-pin that matches the dropped cufflink-- Eddy Duran.
"Gilda did run off with Eddy." Harry was just repeating what Amy had whispered to him moments before. The suspects listened with rapt attention. "But Gilda changed her mind. She left him and came back here, hoping to patch things up with Tony. Eddy followed her. They argued and he killed her. As an ex-headwaiter, Eddy still had a key to the restaurant. He planted Gilda's body and the note she'd left for him, hoping that when the body was eventually found, the police would pin it on Tony."