As someone who plays Smash 99% of the time with GameCube controllers, there are several caveats I must point out:
Bear in mind that a Wii U without a Gamepad is severely crippled. The inability to access system settings includes the inability to adjust for your TV settings, so if the previous owner connected to a traditional tube TV and you are using HDMI, or vice-versa, then you will not be able to see anything. Similarly if the Wii U was last played on a TV at 1080p and your TV only runs at 1080i or 720p, you still will not see anything. Additionally you will not be able to modify Internet settings, so if the prior owner used a wireless setup and you plan on going wired, you are out of luck using online features until you can get a Gamepad to make the needed changes.
Assuming you happen to have the same Internet provider, password, and same type of TV as the previous owner, then I suppose you could get by without a Gamepad. You will still be unable to create Miis, use amiibos, or create / modify custom stages in Smash though, among other things.
I honestly do not know why any Wii U would be sold without a Gamepad. You will need to acquire one at some point, even if you plan on gaming with GameCube controllers almost exclusively.