Of coarse GPS uses data...how else would it be able to locate you?
How much? I have no idea.
But you can most likely compare your data usage from the Settings.app before and after using the GPS.
It's not like it uses a set amount of data...there are too many variables.
Like distance...you will use more data using the GPS to drive 2 hours as opposed to 45 minutes.
It's impossible to give even a remotely accurate estimation of data usage.