I would bet the guided accuracy would be based on initial firing, not on continual in flight adjustment like on a missile system. In other words, they would use GPS calculations of the target to feed into the targeting computer. If they could find a way to have continual in-flight adjustments, DARPA has some s*** way beyond anything I could imagine. Calling it GPS-Guided is probably some reporter’s interpretation by not understanding what some technical description meant.
They already make guided rounds for very large guns...as in the gun on an A10 warthog. The nose of the bullet has a ball on it that will swivel to act like a nose mounted rudder. It swivels fast to keep up with the spin of the bullet.
No, the intention is to actually guide the round in flight.
See this Boeing patent on the projectile:
http://www.google.com/patents?id=_1HGAAAAEBAJ&printsec=abstract&zoom=4#v=onepage&q&f=false
Some old work on a BMD railgun projectile from SDI:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA345781