I think we may be underestimating him abit.Ditto. Let WOLF-MAN have it's own stories for a while first. Besides, unless Invincible has some Superman-esque weakness against the supernatural, a fight wouldn't last long.
We still don't have any true measure of just how powerful he is yet at all. The only thing we've seen Gary go up against is Spore and that obviously wasn't much of a fight. Knowing Kirkman, I doubt he's just an ordinary werewolf. If he can't go toe to toe with powerful super villians/nemesis, he wouldn't be much of a super hero.
Now I'm not yet saying he could win against Mark but he could very well end up being the Kirkman universe's version of the Hulk, except with, you know, brains.