God Mars (also known as Six God Union Godmarz, Six Gods Unit, and [hilariously] Hexademonic Symbiote Godmars) was one of the fairly numerous shows about a big combining robot thingy.
On this side of the Pacific, God Mars is probably most famous among the Japanese die-cast toy circles, and its Soul of Chogokin (a sort of super-high-end collectible "toyline" where average retail price is well over $100) recently won Toy of the Year on one such site.
One notable fact about God Mars is that the original '80s era toy was to have been retooled to become the Mighty Orbots toys. It's just as well this didn't happen-it would have been pretty hideous.
Anyway, the series itself seems to have had more plot than simply BUNCH OF ROBOTS SHOW UP FIGHT COMBINE WIN. The series' main protagonist, some guy, was sent from outer space to Earth as a baby. Why? So that when he grew up, he could use the bomb in the robot that brought him to blow up the Earth!
Of course, this seriously begs the question of why the robot wasn't simply sent to destroy the Earth all by itself. Seriously. But they used this plot point in an interesting way-since the kid is raised on Earth, he loves Earth and wants to protect it (didn't see that coming [/sarcasm]), and so he uses the robot (named, you guessed it, Gaia-wait, that wasn't your guess?) to fight the bad guys. But if he ever dies, the bomb in Gaia will go off, destroying the Earth.
The big bad is fine with this, and tries to kill him, with, among other evil schemes and whatnot, the protagonist's brother who was raised to hate him. But it turns out that the kids' dad, who wasn't totally cool with this whole thing, sent five more robots to help him when he grew up. And these five robots combine with Gaia into God Mars. (The robot that merges with Gaia to become God Mars' torso is strangely called Sphinx, most of the rest are named for planets or moons, one is called Shin, and the last is called Ra.) Eventually, when the main villain is defeated, the villain (who looked suspiciously similar to Darth Vader) exploded into a thousand spores of hate disease or something typically Japanese.
As far as I know, up to the point where this series was released, it was the first robot made up entirely of smaller but still humanoid robots. It was also, to my knowledge, the first combining robot with that many components. The series itself also has some fairly original (in both the good and the bad sense) enemy mecha designs (images can be found on these Italian pages).
It also had a pretty nice theme song.