I played Mario 3D World, Mario Kart, Smash Bros. all on the GamePad.
You might need to have the TV set to the Wii U when you initially boot up as the home screen is a bit weird but once the game is running you should be fine to switch the TV to another input and keep playing on the pad.
There are a few games like Starfox and Zombii U which might not work well since you use the gamepad in conjunction with the TV but like Warbie said most should be fine.
Just check reviews and/or trailers before purchasing if you aren't sure.
The range for me was quite good (due to paper thin new build walls) so was able to play on the GamePad in another room actually.