Here is the text of a posting by one of the developers of GSS (Nigel Bree) in response to whether GSS is compatible with WinPE 3:
Actually, it's perfectly compatible with Win7 PE, and indeed just about any version of Ghost ever is - after all, the only thing Vista did to really break compatibility was UAC, and UAC doesn't affect WinPE at all as you're implicitly logged in as LocalSystem.
Putting Win7 into the management environment isn't that hard either, it's something we simply hadn't committed to and done already in the GSS 3.0 environment becuase Win7 PE still really didn't exist at the point when Symantec shut down development, and so we were getting the GSS 3.0 Beta out before putting someone onto that closer to Win7 release before deciding what we'd do. Most likely, we wouldn't have included it because we wouldn't have had enough time to test it before making it the default PreOS, and in particular it wasn't clear whether it's work any better than Vista in small-memory (i.e., 256mb) machines where we had to really strip down Vista to get it to boot.
So whoever generated this technote would appear to be supplying misleading information. It is true that GSS 2.5 does not natively support USB-3 as USB-3 came after it was released, but the support for USB-3 is ultimately something that the operating system has to provide, rather than Ghost itself.
OK, lets assume that the Iomega drive does not need a driver for USB 3.0. That does not remove the requirement for you to supply a driver for the USB3.0 INTERFACE on your system. You don't mention whether you have added a driver for the USB 3 PORTS on your system to WinPE 2.0.
I can categorically state that I have taken a copy of WinPE 2.0 (same version as is used in GSS), booted a system with USB-3 ports, used DRVLOAD to load the motherboard drivers for the USB-3 ports, then used DRVLOAD to load the driver for my Western Digital USB-3 external drive. At that point, the drive was fully accessible and I was able to use it to XCOPY a bunch of stuff from the external drive to the local hard disk. (I would assume that in your case, the second step would not have been necessary if the Iomega does not require drivers.). Equally, WinPE 3.0 will undoubtedly have additional driver support compared to WinPE 2.0 as it is newer. Nevertheless, WinPE 3.0 is nearly two years old and USB-3 is pretty new, so I would not assume that it will support all USB-3 implementations natively.
As a separate test, have you tried plugging the Iomega into a USB-2 port? USB-3 devices should work quite normally, albeit slower, on a USB-2 port, so if your existing WinPE-2 boot is not detecting the Iomega on a USB-2 port either, then I would consider replacing your Iomega. If it does work on a USB-2 port, then I return to my assumption that you have not managed to load the USB-3 drivers for the USB-3 ports on your machine.