I was about to comment about the AC pass.

As far as how it works, it likely works in one of two (or both ways).

1. When a Wi-Fi computer connects to a network, it's MAC address is unique and is recorded (this is why at hotels you often don't have to reconnect after shutting down your computer within the 24 hour period). When you purchase service, the system knows it's for that MAC address. Regardless, for Gogo this code is a one-time use and is likely linked to that MAC address. Once a device/computer has used the code, the system knows it and won't let it use it again.
2. Gogo logs in with an account. They could mark that account as having used that code and flag it not to work with that code again.

Depending on how they implemented this, it's possible the same user could use this code more than once, but it would require using different devices (another notebook, an iPhone instead of your PC, etc.). If they implement item 2 above, then the user would likely have to use a different device as well as a different account/email.

I used to work for a company that did Wi-Fi for hotels and that's pretty much how it works.