What happens if you plug it directly into the console and open up HID to Vpad? Can you directly have the Pro Controller pretend to be the Gamepad? If not, your solution is the simplest one not to need additional hardware (e.g. an adapter to make it show up as a PlayStation controller &c.).
I hate bumping old threads and this one was active much more recently than the other HID to VPAD thread.
I'm going to try to use this and WiinUSoft to pair my Wii U Pro Controller over my GamePad by using my Pro Controller to emulate an Xbox controller on my PC and using the emulated controller to emulate the GamePad through HID to VPAD.
There's got to be an easier way though, right?