All consumable media, to some degree, is in competition. The average person only has a finite amount of both time and money, which leaves them with decisions to make. After that, it's a matter of breaking down the demographics and seeing how much overlap exists.
Of those Wii U owners who also own a PS3/360, how many are interested in GTA5? Quite a few, I bet. Another factor, I think, is that those who will buy GTA5 are likely anticipating playing that game for a long time, and thus are probably not thinking about other titles. Because of that, I think GTA5 hurts everything around it.
Still, it's going to be mostly the Nintendo Faithful who buy a niche title like W101. GTA5 will affect sales but, because W101 isn't going to move a ton of units in the first place, the actual impact won't be extreme.
