With Gantz though, (with the exception of that one particular "trip") it's a pretty even playing field between the good guys and the bad guys.
I don't think you read the manga 
Either way, the odds against the protagonists have no bearing on how brutal an anime is.
And I don't really get the feeling that humanity can't win in SnK, considering how most of titans seem to have nothing in their heads. It's not that hard for a human to kill a titan one on one either, it's more a question of when they'll start taking their training seriously.
... there's two chapters left in Gantz and I can't wait to finish it.
To me, it has a huge impact on its brutality. The guys in Gantz has access to ridiculous weapons and the power suits. The characters can get out crazy situations... and that doesn't even include the things the black ball can do.
Sounds like you haven't read Shingeki no Kyojin
Regardless, the humans are at such a disadvantage it's ridiculous. They have no overpowered weaponry or anything to protect themselves. All they have is their agility and even that doesn't seem to be enough. If they're cornered or get grabbed, they die. And as you said, most of the titans appear brainless. It's just straight-up, cold-blooded killing.
That is brutal.
