Good luck with that! No guidance system known to man can handle that acceleration and keep on functioning. Plenty of systems with far lower velocity/acceleration have proven impossible to function after being subjected to high g-forces.
Railgun slugs are by definition simple chunks of metal. Even if the electronics could handle the acceleration, they’d be fried by the magnetic field. I think what they’re talking about is how the gun is aimed.
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.
It's perfectly technically feasible to have a GPS guided round, and potentially an active seeker for attacking moving targets - was some work during the SDI days on this.
There have been a lot of advancements in solid-state electronics that can survive high-G and magnetic environments.