Line of sight wouldn't be an issue, range and aiming would be the problem. The GPS satellites are in a bunch of orbital planes so a weapon wouldn't try to rendezvous with each target, it would need to hit them from another orbit. The solution would be to launch the weapon in a lower orbit and just wait until each target comes within range.

If aiming a laser accurately is too tough of a challenge (probably is) then as an alternative you would need 6 weapons and launch each one in one of the 6 orbital planes that the GPS constellation uses and just have a little bit lower orbit than the GPS satellites so it would come around and catch up to each satellite but not be travelling so fast as to make accurately aiming impossible.