Wait so you can use pro controller, wii remote, and controller pro, but no gamepad?
That's odd. Doesn't gamepad use bluetooth?
That would be the greatest thing ever *deep sigh*
The reason why has been explained numerous times by myself in this thread.
In Wii mode, the Gamepad is literally a TV and nothing else. Nothing can be done about it, as the Wii mode does not have access to that side of the console. What happens is that when the console is booted into Wii Mode, a section of Wii U mode is set to mirror the image output to the gamepad, and occupies the gamepad completely.
It is already taken over and used by the Wii U side of things, so there's no way for homebrew to hijack it without then losing the gamepad video stream. Using it for controls on Nintendont is theoretically possible but you can only have controls, not video too. It's one or the other.