Open a command prompt and run syslinux.exe -ma X: (where X: is the drive letter of the USB flash drive).
Verify that the system file ldlinux.sys was created at the root of the USB flash drive (You may need to change the Windows Explorer view options to show 'hidden files and folders' and 'protected operating system files' in order to see the file).
Now your USB Flash Drive should be ready to boot from and run Gparted Live.