Not much is known about the Gizoid line. Emerl was discovered to be the last remaining robot from the line and Professor Gerald Robotnik theorized that he was created by the Fourth Great Civilization.
In Sonic Chronicles: The Dark Brotherhood, the Gizoid robots were developed by the Nocturnus Clan as artificial soldiers around four thousand years prior to Sonic's time, as Imperator Ix's people vied for control of the ancient world against Chief Pachacamac and the Knuckles Clan. Marching out of the factories within the Nocturne citadel, the prototype Gizoids fought at the forefront of the Nocturnus armies, and (according to Ix's hyperbole before the final boss in The Dark Brotherhood) turned the tide of the war against Knuckles' ancestors. Gizoid's strength lay in their ability to flawlessly mimic their enemies' attacks, allowing them to become more skilled in combat with every vanquished opponent.
After Perfect Chaos obliterated their rival brethren, the Nocturnus Clan enjoyed a free (if brief) reign over Sonic's world. Robotics research and development didn't stop during this time; the pinnacle of Gizoid science in the ancient world came in the form of Emerl, a robot with the ability to absorb Chaos Emeralds. As Nestor the Wise theorized, it was precisely this breakthrough development which attracted the attention of Argus, and led to imprisonment of the Nocturnus echidnas in the Twilight Cage. Rightly or wrongly, the Nocturnus were removed from Sonic's world shortly after Emerl's creation.
Excluding Emerl, every single Gizoid under Nocturnus control was transferred to the Twilight Cage in the Argus Event, along with the Nocturne and (apparently) most - if not all - of Nocturnus civilization. Some artifacts apparently remained, and the Nocturnus were dubbed "The Fourth Great Civilization." Imperator Ix continued to use the robots as the echidnas subjugated the otherracesofthe Twilight Cage, and advanced Gizoid models remain a central part of the Nocturnus army when Sonic and friends arrive there during Chapter 7 of Sonic Chronicles: The Dark Brotherhood.
List of Gizoid Models
Gizoid - The basic combat robot employed by the Nocturnus army throughout the Twilight Cage in Sonic Chronicles: The Dark Brotherhood. Has 250HP, and can use any and all Nocturnus and player-character POW moves.
Gizoid Mk. II - A slightly stronger version of the basic Gizoid, with 340HP.
Gizoid Centurions - Advanced Gizoids that ambush Knuckles and Shade's party on the Nocturne. Gizoid Centurions have a protective, starry shield around them which renders the robots totally impervious to attacks. Only Tails' quick thinking enables the heroes to disable their shields, using a specific sonic frequency.
Gizoid Guardians - Elite bodyguards of Imperator Ix, only seen during the final boss fight of Sonic Chronicles: The Dark Brotherhood. These robots have a single, wide wheel in place of their feet, and attack using a massive pole-arm and a signature POW move, 'Crippling Arc'.
Prefect Scylla - The other Gizoid Prefect of the echidna armed forces. Scylla is somewhat mentally unstable, and will often laugh crazily at the end of his sentences. Scylla fights Knuckles and Shade as another sub-boss on the Nocturne.
E-121 Phi - After studying Emerl before the opening of Sonic Battle, Eggman reverse-engineers the Gizoid's mimicry capabilities, enabling him to produce his own series of move-copying robots: the E-121 Phi line. Powered by Chaos Emerald shards, these robots are recurring enemies throughout the game.
G-merl - After Emerl's destruction in Sonic Battle, Eggman used the Gizoid's data to construct G-merl, who functions as a recurring enemy (and final boss) in Sonic Advance 3. Unlike normal models, G-merl seems to lack the typical Gizoid penchant for move-copying. At the end of the game, he is discovered by Cream and reprogrammed by Tails in the image of Emerl, making this robot some sort of tripartite fusion of Nocturnus, Eggman, and Vulpine technology.
In the Sonic the Hedgehog comic series and its spin-offs published by Archie Comics, the Gizoids were a line of super-robots created by the Nocturnus Clan. Incredibly adaptive with an advanced A.I. and capabilities equal to an entire army, the Nocturnus Clan would fall back on their Gizoid army when their own personal tech failed.
After reality was rewritten by the Super Genesis Wave, the Gizoids' history became identical to that of their game counterparts.
In Sonic Battle, Gerald's diary reveals that "Gizoid" is a term in an ancient language for "everything". This refers to the Gizoids' ability to copy and adapt to everything.