They now have a skill gap of mid-seniority navigators who have no astro-nav experience and will struggle under GPS denial.
Easy. The same number that know how to use a GPS. Your one guy may be the first casualty.
We've got plenty of ways to defeat jamming, the newest block of GPS satellites have ways to increase signal power in certain areas and anti-radiation missiles can blow up a jammer.
What do you do when your enemy dumps a load of sand in orbit?
That's about 4 or 5 people on a typical warship, and then you can add to that pool the number of people who did hold a job that required the skill but have since been promoted.