While the Giant Robo OAVs have only been around since 1992, the story itself goes back a good deal further. There was a live-action version in the sixties (?), released in the US as 'Johnny Socko and His Flying Robot'. The basic structure of that show was the same as the current anime: a young boy who controls a giant robot joins an international organization to fight evil. Some of the episodes were re-released on home video last year by Orion.
One of the interesting things about the Giant Robo OAVs is that many of the characters are pulled from other projects by Yokoyama Mitsuteru, the creator of the series. There is, of course, the Giant Robo manga; other sources include Babel II, and Yokoyama's adaptations of the classic Chinese stories Tales of the Water Margin (Taisou and Tetsugyu) and Romance of the Three Kingdoms (Kumei). Other characters are stock personas Yokoyama uses in many stories, while a few, such as Ginrei, are unique to GR. I, personally, have seen Babel II, and must say that I agree with things people have said about how OAV director Imagawa Yasuhiro's use of certain characters raises some interesting questions--such as who, exactly, are supposed to be the 'bad guys' (an issue which becomes more prominent in 6).
Note: Some of this information was gleaned from postings to rec.arts.anime, and is certainly not guaranteed by me to be completely accurate. What I have here is what seems to be accepted by most of the knowlegable people on the group. Sources include Enrique Conty, Michael Liu, and David Crowe, who are all more than welcome to send me irate e-mail if I've misunderstood the facts. *grin* I would also like to thank John Chambers, aka Atmavictu, who corrected some of my misunderstandings.