The app doesn't label itself as GPL licensed... The terms of the installed Android app are clear that it's closed source [0].

There's a community edition that's GPL, and it does say they're 'going open source' but clearly it's not the exact same app as the official distribution:

  This is the repository for the Chatbox Community Edition, open-sourced under the GPLv3 license.
  
  Chatbox is going open-source Again!
  
  We regularly sync code from the pro repo to this repo, and vice versa.