Why in gods name would you ever voluntarily want to use that piece of shit?
I can barely muster the strength to use it when my Wii U forced me to. It's such a poorly designed piece of crap compared to almost every other controller you can find on the market. Even a PS3/4 controller is better.
Anyway to unsync the only answer I saw on google was "Home button, controller settings, pair controller, then just cancel, it'll unpair everything except the Gamepad."
I have no clue if it works because my WiiU is...somewhere. In a suitcase I haven't unpacked from 4 months ago i think. So I can't bother to test if that's actually legit advice.