Vigil Games has confirmed that the upcoming Wii U port of Darksiders II will support playing the game on the GamePad screen, so you can take your game with you to the bathroom, in bed, or wherever you want (within a recommended 8 metres or so from the Wii U console).
If you're using your TV, however, the screen on the GamePad will be used for displaying the map and giving you easy access to your menu items.