Gaiking was one of the first generation of super robots.
Its notability derives from two main factors: It's one of the first transforming/combining robots with a transformation that is based on physical reality, and it's terrifying.
Seriously, he just straight up uses that giant mouth in his stomach to bite things and those horns on his head to impale things. When I first realized that, I was pretty amazed that something quite that freaky was part of the old guard. (Granted, it's a slightly tame freaky, since, you know, robot-on-robot bloodless violence, but still.)
It actually got dubbed and put in that old standby of super robot lineups that very few people in the US other than hardcore super robot fans remember, Force Five. (It was also a part of Shogun Warriors, although it didn't appear in the Marvel comic.)
Since I don't hear anything about this dub, I'm going to assume it was pretty forgettable (instead of rampantly bad like the Grendizer dub).
I might not even bring Gaiking up at all, except...
...yeah, it had a popular remake/not truly a sequel. And having watched, oh, six or seven episodes or so of its official Hulu subtitled release, I can verify that it's pretty awesome.
For one, they take many super robot ideas and many "real robot" (i.e. what an English speaker would call a "Japanese mecha" series) ideas and blend them together in seamless and logical fashion. Sure, the robot's a super-awesome gigantic thing powered by its pilot's magic fire-generating abilities, but it also needs to have its forearms replaced after it launches off its rocket punches.
Another thing is that the protagonist, Daiya, is perhaps the best iteration of the overused and often poorly utilized "kid pilot" trope found in this kind of series I've ever seen. Seriously, he's just an awesome kid. He's believably a child, yet he's also surprisingly mature, serious, and tough. And willing to take on giant monsters armed only with a probably homemade harpoon.
The series is 39 episodes, and I'm looking forward to all the ones I haven't watched.
Oh, yeah, and there's apparently going to be a movie?
(A little note on Gaiking: Go Nagai has taken credit for creating Gaiking, but he waited to do so publicly until after the popular recent series. I find this just a bit suspicious, especially since Go Nagai seems to kind of just let people assume he created Getter Robo when he really only gave the actual creator a suggestion to go on. Not to call him on it or anything, it's just a bit peculiar that he'd wait until it was clear the property was viable.)