Why do CEF and GTU carry such a premium over Net Asset Value?
Because this is the premium people are willing to pay for what they perceive to be 'real bullion' versus 'paper bullion.'
GLD tends to have a negative premium because it is a vehicle for traders, and particularly short sellers.
Deviations from NAV in themselves mean little, if the deviation is steady. It is in the fluctuations that information, and opportunities, are discovered.