Like I said, it has nothing to do with thinking he's not good enough or hasn't been playing well enough in certain games...it's the need to constantly point it out, or the want to laugh off opinions that are supportive of the player, just because you have a different opinion. Why not respectfully disagree while having an element of respect for the ongoing support?
And people do have vendettas about United players, sadly. As I said it's not necessarily your own fault but it builds up from repeatedly arguing against someone who you perceive to have an extremely and unjustly positive view about a player, to the extent that everything he does is given extra credit just because it's him, and you then take it upon yourself to attempt to balance this and in doing so create a consistently negative and vindictive opinion yourself, which I think translates to a vendetta.
I'm not sure anyone does have a vendetta against Gibson, he doesn't seem typical of the type of player that normally receive one, but it is getting to the point where people seem to enjoy him messing up just so they can get one over on the supporters of him. I know people enjoy being proven right and all I just think when they do so about a player failing it's worryingly sadistic and a bit of a shame.