I'll try and clear it up. I'll assume that you at least know about Getta Robo, Getta Robo G and Getta Robo Go.
Ok so Armageddon is a 13 part OVA thats set in the future, it was made in 1998 as far as I know.
And Shin Getta Robo is a remake of Armageddon but the story is told slightly differently with some different outcomes, and this one is 26 episodes I think. So essentially its a retelling of the same story but witha few things different
(BTW I could be mistaked and armageddon might be the remake, can't remember exactly, but I think I'm right.)
I kinda know about those others but I don't know anything about their stories or anything. Are there HK's of them anywhere? I still think whoever brought over armageddon without the original series first needs to be fired and black listed from the anime business. Stupid, stupid people.
Also, in martian successor nadesico is the show they watch in there (gekigengar or something) supposed to be getter robo or making fun of getter robo? The robots look remarkably similar.
Ok basically during the 1970's a man named Go Nagai created a series of giant Robot Shows, famous things like Marzinger Z, Getta Robo, Grendizer, Gaiking etc.. As far as I know he created the arms firing off from robots. Because these shows were super popular there were many similar shows released. Thus it became a sort of "monster of the week" super robot show show genre.
Gekigengar is basically slagging the whole genre, but its mostly slagging the original Getta Robo. Its not a real show either, some people get confused because there was a Nadesico anime OVA released called "Gekigengar" and basically the guys from the show go to the cinema to watch the gekigengar movie.
As for the Getta Robo's hmm.. ok in the 1974 I think it was, the original Getta Robo was released, it was the 1st ever combining robot show. It never got a western release. The next year in 1975 a sequel series was made with the same characters, but a more powerfull robot was in it and a new enemy, some nazi looking aliens with horns. That was Getta Robo G it was a direct sequel to Getta Robo. This was released in a super edited form in the US as part of a "Force Five" series of animated giant robot shows, this was done in the mid 80's I think. Oh the US version was called "Star Vengers". This does not have a HK version yet, but I'm confident that one will appear because it was re-released late last year on DVD in Japan.
Then in 1991 there was a show released called Getta Robo Go, which I don't know alot about, it is the rarest of the Getta Robo's. Aparantly its some kind of re-telling of the original Getta Robo only with new pilots and now Getta 1 has a cool looking cloak. This also was never released in the west and has no HK version.
If you want to know more about any of this or the genre in general then please ask
Can I get the MSN OVA's on HK or R1? Does it have an R1 release?
MSN? = Missing?? I'm not sure...
Hmm, I think ADV have released or are in the process of releasing both of them but I'm not sure, you may have to get the HK of the ones your missing.
MSN = martian successor nadesico
Oh, sorry. My mistake. As far as I know the OVA does not have an R1 release, but I'm sure there is a HK of it somewhere, but I have never seen it around any online shops. Maybe you can request it from JJ?
I've seen Getter Robo G, and as far as I know that doesn't help understand Armaggedon in the slightest. Either some of it was in Getter Robo Go! or...it's just not meant to be known. I actually didn't have too much trouble picking up a lot of it as the series went on...the characters look very different than their old 70s versions anyway so I had a hard time placing a lot of them. The point is, I really don't think the earlier series would help the situation much.