The gTablet has a 1.3MP camera, the Xoom has a 5MP camera. The gTablet has a 1024x600 screen, the Xoom has a 1280x800 screen.
IMHO, the specs of the gTablet are more in line with the entry level iPad, not the Xoom.
However the hardware is great and 3rd party developers are supporting the device with proper software. You can currently run CyanogenMod7 (which is based on Gingerbread/2.3) on the G-Tablet, though it is fully a community project that Viewsonic has nothing to do with. I'm sure the same will be true of Honeycomb soon after it hits full ASOP status.
If you're willing to hack around with the device and not rely on the official software, the G-Tablet is great at <$400. My only complaint is the viewing angles on the LCD aren't that great (the panel is not IPS), if not for that it would be a pretty ideal Android tablet platform for people willing to install community roms.