To run gamecube or wii games there are 2 ways.
FIRST, vWii mode:
1. Use an usb conected to the back (use the one above, i tried others and doesn't work , the usb must be formated in fat32 with the wii config, and can be, hdd with externa power supply, flash drive or micro sd card with usb adapter but not all adapters works) or an SD card.
2. your vWii must be flashed with ios and that, i recomend use wiiflow, you cant use wii u gamepad in this method, it's like playing on a wii.
ADVANTAGES:
-Have wii, gc, and other emulators on the same drive
-you can manage files on the pc
-use your favorite homebrew apps or laucnher
-everything that comes with wii flashing
-Doesn't need an sd card connected o cfw
SECOND, directly install with wup installer.
1. You need free space on internal memory or and usb formated to work with the wii u.
2. You need to convert every wii or gc game and install 1 by 1. ( i use TeconmoonWiiVCInjector but there are others.) and SD card to install games, once installed, SD card is needed to play only gc games, wii games doesn't need it.
3. You can use gamepad with the gamecube games and wii games that support wii classic controller(the ones doesn't work with classic controller will work with normal wiimote), also you need to config nintendont (use TeconmoonWiiVCInjector too) on the sd card and have it on the wii u to play gc games.
4. You need to launch cfw everytime you want to start a game
ADVANTAGES:
-Play all gamecube and some wii games with gamepad
-Install games on the same drive as wii u games