Minecraft – play Minecraft only using the Wii U gamepad

According to the Skylanders Giants Wii U manual:

CO-OP PLAY
At any time during Story Mode, a second player can join the game by hitting the A Button on their Wii Remote, and placing a Skylander on the Portal of Power. To drop out of the game, Player 1 or Player 2 must remove their Skylander from the Portal of Power and press the B Button on their Wii Remote.

(Emphasis mine).

I would assume what happens is they don't have two Skylanders on the portal, so they just switch control on a single Skylander between the two controllers.

To play Wii Only games it would kind of work if you have a sensor bar, but several games require the gamepad either somewhat (it's required for some levels in Super Mario 3D World but might not be required to reach the "final" boss) or entirely (zombiU and Nintendoland are unplayable without one), and the eShop is completely unusable without the gamepad. I really wouldn't recommend it without the gamepad unless you're using it as a Wii replacement.

Also I'm not sure if first-time setup requires the gamepad (not willing to factory reset to test), but the Settings menu requires it, so for sure you won't be able to set up Wi Fi, download system or game patches or access the digital game store ever. It's really not designed to be used without it and while it technically does not need the gamepad for all uses, it's really not a properly functional device without one.