The answer is hidden under their "inflight texting" information:
One free hour of data per flight:
On all Gogo-equipped domestic flights, you get one free hour of Wi-Fi to surf, email, and post to your hearts’ content.
Streaming services for music and video are not included.
Link to page itself:
https://support.t-mobile.com/docs/DOC-14943
The question is how long this will last?