How is Gatsby foolish when it comes to Daisy?

Gatsby is foolish when he believes Daisy loves him in the same way he loves her. His love for Daisy is all encompassing. He has dreamed of her for five years and arranged his life with the idea of meeting her again and starting over as if no time has passed between now and when they first met. He buys his mansion to be near her and throws lavish parties in the hopes that one day she will show up and be impressed.
Daisy does say she loves Gatsby and even agrees for a short time to go off with him, but hers is a far shallower, more superficial love. She has an affair with him, in part, to get back at Tom for cheating on her. When push comes to shove, her deepest desire is to stay with Tom.
Gatsby is foolish to think she would buy into his fantasy that they could resume their relationship as if nothing had happened. She has moved on by marrying and having a child. He is the one who is stuck in the past.