Are you talking about a USB stick or a USB hard disk?
USB sticks are normally formatted as FAT32 so you are limited to ghost images that are broken down into 2Gb files. If your Ghost image is a single file greater than 2Gb you will need an NTFS formatted device.
Are you intending to make the USB device bootable? If so, PCDOS can only handle FAT32, whereas WinPE (recommended) will handle both FAT32 and NTFS file systems.
I believe GSS is able to make bootable USB media so you should be able to do exactly what you want.
I personally use a bootable WinPE USB device that I have made using the WAIK and then use an old version of Ghost (V8) at the command prompt, and that works fine for me.