TMI - I'm not GPS expert, but it would surprise me if a military system had no ECCM capability by design. Perhaps it's not in the public domain, even if it did.
The other thought I had is in response to the question, why the enemy would want to jam it at all, since it would be useful to them too. Why not introduce a widely variable (10s of km) error, but have the error factor known to own forces? Nice bit of disinformation warfare. Much more sophisticated than jamming.
I believe we retired our (ground) jammers, because they were as useful as a 10MT nuclear warhead on the tactical battlefield.