A bit late reply, but since no one has answered the question yet...
The nüvi 310 (any nüvi 3xx in fact) is not capable of using files larger than 2 GB. The gmapsupp.img file size of GB Discoverer is 2.47 GB. So this map won't work on the nüvi 310, no matter how you unlock it. The unit will simply not show it.
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