Higurashi no Naku Koro Ni (the first installment of When They Cry) is actually less gory and more focused on a slasher/thriller story.

Gantz is made for gore. Pretty much made for gore, period.

An odd choice, but notable indeed, is Neon Genesis Evangelion. The beginning is pretty lackluster, but the end(ings) are pretty gory. The amount of gore is relative to which ending you see.

Space Runaway Ideon. Perhaps the one other reason why Yoshiyuki Tomino is known, aside from making Gundam. Total body count: one Universe. And the Ideon is still around in the Gundam universe...

The first episode of Soukou no STRAIN is pretty gory, but for different reasons. You see the starts of a cheery manga, you can pretty much determine the outcome and think "this must be a romantic story with mechas..." And all of a sudden, a mid-episode massacre. Of course, not that gory afterwards, but it pays to mention.

Uzumaki is yet another gory manga, on the same lines as Higurashi. It has to deal a lot with spirals...and suicides...and bloodsucking pregnant women...well, it's a thing to see.

Bokurano. Just...Bokurano. Remember when I said that Tomino had one entire universe destroyed? The mangaka of this story has around 32,678 parallel universes destroyed...not to mention all of the children that pilot the mecha. Oh, did I mention it's a Super Robot anime?

I guess that's all. Did I miss one, by any case?