GNAV is actually one of the subjects, where you learn something useful for once. Yes, you're very unlikely to do any manual calculations like that in a modern jet, but it gives you good foundational understanding. As for the exam itself, I'd say time management is key. You get aprox 2min per question and you really have to fit yourself into those limits, otherwise you'll run out of time. So, learn to do the calculations fast and practice a lot. Good luck!