You can see how much data each app (and iPadOS) is using from Settings > Cellular Data.

If you want to know how much data you are using once you start using your GigSky plan, you can reset the counters as soon as you buy your plan by going to Settings > Cellular Data, scrolling all the way to the bottom of the screen and tapping Reset Statistics.

You can see how much data you have left in the GigSky app: My Plans.