YouTube videos that are played through the BTS (BOSS Tone Studio] software for the Pocket GT App (iOS/Android) can be viewed in Full-Screen Mode. Full-Screen view is shown if/when the Vertical/Portrait Mode in your device switches to Landscape Mode.
"Session View" in the BTS for Pocket GT App allows you to view a YouTube video in Full-Screen when the iPad is held in the Landscape Mode position. If this occurs, physically turn the iPad vertically so it is in Portrait Mode. This will return the Session window to normal viewing and you can further navigate within the app.