If you (a superpower) deploy a bunch of GPS dependent equipment, and I (another nuclear power) am fighting you--and I'm less dependent on GPS--why would I not destroy GPS immediately, given that it's not very hard?

EDIT: Destroying GPS completely with nukes lobbed into space isn't the only way to mess it up either, it's susceptible to more terrestrial electronic countermeasures as well. It's generally not a good idea to depend on GPS alone, you need other backups as well. Pilots know this It's why they fly with a paper chart and a compass as well as GPS.

EDIT: Also, if my opening move is nuking space, I don't just deny you GPS, I also deny you all your other space-based advantages. I'd guess that would be one of the very first things any nuclear adversary of the US would do, because the damage is so asymmetric.