If you've seen all of Giant Robo episode 7, then you know that there are some hints that the Giant Robo saga continues even past 7. For example, at the end we see the real Big Fire awaken from his sleep, and a final "confrontation" called "The Seige of Babel" is mentioned (dub version). Alberto, right before his demise, also hints at a final battle between Big Fire and the "IPO vigilantes".
From what I understand, "Operation Earth Stand Still" was only one segment in a chain of stories, including "The Domino Operation" and "Birth of Zangetsu the Midday". These other plots allow for many new stories in the Giant Robo universe, past as well as future.
Also at the end of episode 7, there is mention of the "GR Project". The GR Project is an important and mysterious background story of sorts. It seems to involve Giant Robo, but we are not sure what sort of purpose Robo is supposed to serve for Big Fire. Komei mentions that the countdown has already begun. Countdown to what? The view shifts to an ominous dark face shot of Robo, and then a big light appearing on the side of the Earth, as Komei laughs maniacally.
I have heard that in the story of the original Giant Robo manga by Mitsuteru Yokoyama, a scientist wanted to take over the world using three robots: GR-1 (our familiar Giant Robo) for land attacks, GR-2 (appeared in episode 5) for aquatic attacks, and GR-3 (seen in the OP for episode 2) for air attacks. A boy named Daisaku Kusama got control of GR-1 and used it to combat the other two Giant Robots.
Do GR-2 and GR-3 have relevance to the GR Project? Did Big Fire have the three robots created to take over the land, sea and sky? According to rumor, GR-1 is the only nuclear-powered robot. GR-2 and GR-3 use some other power source, and this is connected in some way to the GR Project. As Alberto once said, "We can only wonder at their true motive."
Giant Robo: The Animation also has some pop culture references. You may know that Greta Garbo was a Hollywood actress starring in films of the 1920s and 30s such as The Torrent (1925), Anna Christie (1930), and many others. Greta Garbo is also the name of the airship used by the Experts in Giant Robo.
JigenX sends the following: "It's a little known fact that Franken Von Fogler's name is based off of [The Rocky Horror Picture Show's] Dr. Franken Furter (because Robo's director loves the film) and in the opening animation, there is a building that's supposed to be the Great Northern Hotel from Twin Peaks ( I believe it's in the first laser sweep or that one building that gets sliced)." There are probably even more cultural references to find.
Here are some GR rumors that visitors of this site have sent in. It's interesting stuff!
"I heard that GR-3 was destroyed when Robo fired his bazooka (through GR-2's head). The base that was destroyed also held GR-3, thus the huge explosion. The GR project (in this case referring to the plan to create the three robots) was then deemed a failure." --from Kel
"Incedentally, did you catch the reference to Zangetsu's origin from the liner notes translation you link to. I really think he's Kusama Disaku's brother." --also from Kel (ed: that one just makes my head spin O_o)
"I am sure that Kenji Murasame is still alive even after his gruesome demise in ep. 7. I also heard that the Big Nine come together to combat Big Fire." --from JigenX
"Koenshaku is supposed to be the ghost of Koushin's dead brother, and only Koushin can kill him." --from Steve
"...What if GR stands for Ginrei? Komei was obviously totally prepared for Ginrei to teleport Ryu-zimpaku to New Abbay, so could it be part of his plan to harness her power, or the side effects of the power to their purposes? What if the GR title is to purposfully misdirect you into thinking it's Robo related, like the fragments of memories in the first couple episodes that suggest Fogler is a madman which turn out to be false?" --from Charlie Wong
"I believe that the two titan robots at the end of the series [Neptune and Galuda] were in fact GR2 and GR3 or at least the machines created to battle the sea and air...Perhaps the Siege of Babel is the battle between Robo and those two titans!" --from Jon Norwood