As I recall, the Army was working on GPS-guided munitions years and years ago. The rationale was that more accurate artillery would lower the logistics supply burden and also reduce collateral damage. Developing longer range artillery instead of using SRBMs also addresses several issues with treaty monitoring, interservice agreements on mission allocation, etc. As was pointed out, testing could be a little bit of an issue but they could probably use radar to track the projectile and verify its trajectory.