GPS doesn't use data per say, unless it's AT&T's where it downloads maps on the fly. With something like Navigon, the only data usage would be looking up POIs with Google, and that is very low data usage.
I am on the 200 plan and use GPS all the time. I end up using less than 100 MB and often less than 50 MB.
YouTube is another issue.
This, GPS will just consume data if you load maps on the fly, like Google Maps. However, Google has such great compression algorithms that maps render very fast with very little data. That could be seen and experienced on the 1st Gen iPhone that was EDGE only and still Google Maps loaded *very* fast.