Eventually, GLD should be near worthless as the physical stocks backing it dry up. Conversely, the premium that sellers can charge for the physical will rise as supply diminishes.
We will then have a better idea of the true value of physical.
Actually, if the price of GLD falls faster than that of physical gold, the ETF's managers buy GLD shares and finance that by selling their physical stocks of gold, thereby increasing the supply to retail buyers. The point of this activity is to increase GLD's share price so as to bring it in line with that of physical gold.
When GLD's price rises faster than the price of physical gold, the ETF's managers do the reverse - they issue new shares of GLD, depressing GLD's price and use the funds to buy up physical gold, thereby increasing the price of physical gold. The whole idea is for GLD's price to track physical gold all the time.