Charles,
It's true that the GPMC has a max. 512MB address space. However NAND memories are not memory mapped. They are controlled by a command/address/data interface that's transmitted over the data lines. The GPMC can handle both 8-bit and 16-bit NAND devices. And yes, theoretically up to 7 NAND devices of different sizes can be connected to the GPMC, although in real life this would depend on whether all 7 chip-selects can be available through pinmuxing. I haven't checked, but some of them may be overlapping with other necessary signals.