Just like any other app, when you click Sync in iTunes, the app is backed up and encrypted and hidden away in somewhere in the directories of your computer. You cannot view the data they contain... well, at least not easily or without a lot of tech know-how.
If you're asking about Online Sync (not offered in Gas Cubby by FRAM right now), tapping Sync on the main screen of Gas Cubby sends your data to a database where it's encrypted and only accessible through the Sync and Delete Sync Data buttons in Gas Cubby; it is not accessible through a web browser.
The only way to access your data on your computer is by exporting it from Gas Cubby using the feature aptly called Export.
http://www.appcubby.com/gas/manual.html#exportIf you downloaded Gas Cubby by FRAM, you'll have the following icon in your apps screen:

Your screen will have the FRAM ads on the bottom and the Transfer button in the lower right corner.
