GPS alone doesn't use any data, but most likely your app will. Most tracking apps communicate with their servers back and forth saving your data, getting weather updates, etc... Also if you are looking at the map as you ride, it uses data to pull down the maps its showing you. Overall it doesn't really use much data, but it will use some.

You can track how much data it uses by just going into Settings -> Data usage before you ride, then after and noting how much it used.

FYI the only really nasty stinkers on data usage are things like Netflix, or Radio apps like Pandora, or watching YouTube videos, other than that 2 GB a month should be plenty even if you ride a lot.