So you freewheel once GPS goes down. You might drift a few microseconds. This is not necessarily a problem.
If you're traveling at high speeds and relying on this timing to adjust flight control surfaces, or trying to use radar to get position of combatants and/or the ground, a few microseconds clock skew would be a major issue.
I didn't say clock skew. I said drift. As in, you lose GPS timing signals so your own internal master clock freewheels. That has nothing to do with clock skew (different components within the jet 'seeing' different time due to delays or other issues). The jets are designed to function with degraded or no GPS functionality. But, if it's there, they will take full advantage of it.
For those downvoting, see my comment below, where I explain why this really isn't a problem.