why did gatsby change his name

Gatsby was out warning the rich folk to get their boats off the lake because a storm was coming in... and when asked his name he said it was Gatz rather than Gatsby. He wanted to sound like one of the crowd or possibly disassociate himself from his poor family.

That answer too^ and also because he wanted to create a new life for his self. He hated poverty and wanted his imagination to come to life. Once he got wealthy he changed his name and lifestyle