Like he points out, it is logically consistent to think that nothing can be justified except that nothing can be justified (this isn't self refuting, just poorly worded by me). At times, people don't attempt to justify any beliefs, only use instinct to act, don't reason about anything. Does that mean they never do?
If you have beliefs, you would be hard pressed to convince me that you cannot justify any of them and that doesn't matter to you, let alone anyone else. The belief the GPS is working, tacit or not, would amount to something different if you couldn't justify it, tacitly or not. You'd check it more, anyway. You might reply that you instinctively suspect that it's stopped working, cannot justify the claim that you are going the right way, but
some people class reliable processes as justifying ones
A functioning GPSis reliable, so why doesn't it justify - to you or objectively - your belief in your location?
Two quick examples of reasons to think that it is psychologically implausible to claim you cannot justify anything. It may not be certainly so, but then philosophy rarely deals in certainty.