UFO Robot Grendizer vs. Great Mazinger is a short animated film featuring the Grendizer and Great Mazinger based slightly on a few chapters of the original Grendizer manga. Like other Vs. movies, it is non-canon to either series.
After numerous failures against the Grendizer, King Vega sends General Barendos and his troops on a mission to Earth. Before landing, Barandos takes the opportunity to gloat at Blaki and Gandal that if he succeeds in his mission; they will prove to useless and killed. Using the Saucer Beast Jinjin, Barendos captures Koji Kabuto in the TFO. Koji is then brainwashed to reveal the locations of the Mazinger Z and Great Mazinger. Taking the Great Mazinger, Barendos pilots the Great against the Grendizer. The giant robots clash for a while until Koji tells Duke Fleed about a place to deactivate the Great. Duke complies using a special shot, which allowed Koji to use the robot after Barendos escaped it. The Grendizer then proceeds to destroy the mothership Barendos is in. With the threat stopped, peace had returned.
Grendizer vs. Great Mazinger uses materials from a couple chapters in Go Nagai's Grendizer manga. The use of brainwashing was used by a soldier of the Vegan Empire to take control of both Koji and Tetsuya Tsurugi for them to pilot Z and Great. Barandos himself appeared in a chapter, taking the Great Mazinger and a few Mechanical Beasts in a battle against the Grendizer.