I recommend going to Menu - Initial Setup - data acquisition section - and click on the data source button. Is the GP330B set as preferred ? Does it show up under current ? Do you see a lat/lon to the far right ?
If it has clear view of the sky; it appears that something is wrong with your GP330B or the CANbus supporting it. You might want to look over the wiring and the unit to see if you can find the issue or contact your local Furuno dealer.