You can see how much data each app (and the iOS system) is using from:
Settings > Cellular Data > Cellular Data Usage / Use Cellular Data For
If you want to know how much data you are using once you start using your GigSky plan, you should reset the counters as soon as you buy your plan by going to:
Settings > Cellular Data > Reset Statistics
This button is all the way to the bottom of the Cellular Data Settings area, beneath the fields that show how much data each app has used.
You can see how much data you have left of your GigSky data plan by looking at the dashboard in your GigSky Account:
Settings > Cellular Data > Manage GigSky Account...
The remaining data is also shown in the iOS settings under:
Settings > Cellular Data > Choose a Data Plan
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 Manage GigSky Account.