If you hit "TV Only" and boot into Wii Mode, you can enter "TV and GamePad" mode by turning the GamePad back on.
The first time I tried using the GamePad mode, I didn't know about the on-screen menu. The wife was watching something and I wanted to check it out, so I booted the Wii U, selected Wii Mode, then used my Wii Remote at the TV screen until I rumbled over a button and just hit that button. When my GamePad turned off, I was all sad that it didn't work, but then, I just tried turning it back on... and viola!