GPS is well above where Kessler Syndrome would be a problem. GPS orbits in medium-earth orbit with 12-hour period. The only other thing in area are other navigation constellations and there is much more space than LEO. Also, GPS is high enough that would require special and large anti-satellite weapons to take out.
There was a post this week, https://news.ycombinator.com/item?id=36043377, about alternatives to GPS.