My concern is more that a GPS guided weapon, whether a smart bomb, a cruise missile, or an incoming ICBM warhead could already be systematically diverted away from its target.
No one to look out the window on any of those platforms...
I think the best that could be expected in that case would be for such a weapon to miss its target. It would be very nearly impossible to send out multiple time markers for the purpose of substituting a current fix. Coordinated microsecond precision between multiple markers would be needed to spoof any fix, especially since the position of the target being spoofed would have to be known to within a meter. Besides, the timer markers, ephimeris/almanac data for the military is seriously encrypted and transmitted with spread spectrum techniques.
I don't think GPS can be hacked. It could certainly be jammed.