"A love-tap? If Your's is a Megadeus, then mine must be a Gigadeus!"--Beck
The Gigadeus was a large megadeus that appeared in the manga. It was piloted by Beck, who recieved it from Angel. After initially dominating the fight against Big O, Beck eventually started spiraling out of control into insanity caused by the Gigadeuses influx of memories and hallucinations, distracting him long enough for Roger to use his Chrome Buster to destroy it, seemingly killing Beck in the process.
The Gigadeus was a 3-headed tripedal giant robot roughly twice the size of Big O. It's coloration is unknown due to being a manga exclusive. It has a streamlined yet bulky aesthetic similar to that of the big-type megadeuses, such as the heftier shins, the gigantic forearms, the spiky hip section, numerous bolts and hole patterns, as well as some of it's own unique details, like the ornate chest area. It also bears some features resembling the archetypes, like the big empty holes in the heads with pin-prick lights at the center. It's 3 heads are shrouded inside of the 3 large horn-like structures atop it's shoulders, resembling a crest. Each head inside is the size of Big O's head roughly, and they can emerge from the horns like serpents, with tentacle-like "necks". Each head has it's own distinct design, the right head having no eyes, the central head having no mouth, and the left head resembling a dragon. It has 2 massive drills at the back of it's forearms, and 2 giant sawblades under it's "crest" and above the shoulders.
Judging from at least 2 of Gigadeuses 3 heads, Gigadeus seems to be based off pictoral maxim known as "The 3 Wise Monkeys" and the phrase "see no evil, hear no evil, speak no evil", and it's western interpretation to representing immorality, irresponsibility, and ignorance, which perfectly coincides with Beck's behavior, the "wisdom" of Gigadeus and it's memories, and the western style of Big O as a series.
Abilities and Traits
Gigadeus was heavily armed with the previously-mentioned drills and sawblades, and also featured weaponry on it's heads. The central head has a set of lasers that can shoot out, and were powerful enough to overwhelm Big O's standard eye lasers. Then his left head possesses fire breath. It's unknown if the head at the right has any special abilities.
The Gigadeuses armor was impressively durable, easily withstanding both Sudden Impact and Missile Party from Big O. It also possesed lots of brute strength, being able to kick Big O into the wall with it's central leg, and also accidentally destroying it's left hand when it missed a punch on Big O, ramming it into a wall and completely destroying it. The only weapon Big O used on it that was effective was the Chrome Buster, which blasted Gigadeuses upper half into pieces.
The most peculiar feature of the Gigadeus was it's ability to show memories from 40 years ago en masse. Unlike in the anime, where memories prior to 40 years ago usually consist of the megadeus onslaught during The Event, Gigadeus exhibit's memories of world history. The evolution of flight technology, the history of warfare, and even space programs. There doesn't seem to be any limit to what memories the Gigadeus possesses, and they seem to only come out as per Gigadeuses own accord. It's unknown if these memories are able to appear when above ground, as, according to Beck, Gigadeus allows him to "see" the memories underground.
Unfortunately, there's 2 negative side-effects to being the Gigadeuses' pilot: First, the overwhelming amount of memories become addictive according to Beck, and it's easily seen as his personality shifts dramatically, from apathetic and empty when on the surface (instead of his regular cooky demeanor) to completely manic and insane when piloting the Gigadeus. Secondly, and perhaps tied with the insanity, morbid and disturbing hallucinations seem to follow after the waves of memories, depicting death, corpses, and disturbing imagery, though this is mostly seen at the end.
Only 2 people are confirmed to have experienced these memories from Gigadeus: Beck (who got repeated exposure until the point of insanity) and Roger (during his fight with Beck and the Gigadeus). Lastly, although this has yet to be confirmed, it's possible that Gigadeus was somehow responsible for Beck's return as a paranormal entity that haunts Roger in "Big O: Lost Memories" Volume 1.