Don't know for sure how GPS works, but I assume it is it's own info gatherer independent of data (coordinates, speed, alt.). However, any other mapping information - roads, names, sat images, traffic info, layer info, all graphic stuff not already cached... anything pulled from Google would be data and that uses your data plan. I would think that the GPS info is pulled directly from the GPS satellites and mapped within the Google map program on the phone to place you in the right spot. Someone correct me if I'm wrong about that.
BTW, your title says BlackBerry Maps, but the question is about Google Maps. Two different programs.