Are you trying to use your gamepad as a second screen while you emulate Wii U games on your PC? Unfortunately, I don’t think that has been done yet. I believe the Wii U uses Wifi to send video to the gamepad and no one has made PC software to handle that yet (to my knowledge).
I had heard of people using the Gamepad to mirror the game playing on their PC.
If you are trying to use your gamepad as a second monitor for Wii U games you would probably have better luck getting a Steamdeck (Youtube link may contain helpful comments).