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 Settings > Cellular Data: Cellular Plans. Here you'll find an entry for your GigSky Data Plan with the balance shown underneath the plan name. However, there is sometimes a delay in communication between GigSky's servers and Apple's servers, so the most accurate figure for the remaining data balance will always be found under Settings > Cellular Data > Manage GigSky Account... > Account Settings.