GasTracker is a screen-scraping app that allows you to view and add data to your vehicle's profile on Fuelly.
GasCubby will soon be the native Fuelly app but is currently being reworked to better fit with Fuelly.
More info on GasCubby can be found here on the announcement page:
http://www.fuelly.com/forums/showthread.php?t=1226
For immediate access to Fuelly, you'll want to use GasTracker, but in the long run you'll likely want to replace GasTracker for GasCubby once we've integrated the app with the main site.