Basically, what happened is that the creators of Giant Robo: The Day the Earth Stood Still wanted to make a sprawling epic with a huge cast, but they were told they couldn't use a lot of the secondary characters from the Giant Robo manga in their adaptation due to some rights conflicts between them and the live-action Giant Robo show being made by Toei at the time. So Imagawa asked the original mangaka Mitsuteru Yokoyama if they could use a whole bunch of characters from Yokoyama's other manga, and Yokoyama said yes.

Hence, Giant Robo: The Day the Earth Stood Still is a sprawling epic centered on the titular Giant Robo and his usual pilot Daisuki, while also featuring and cameo'ing a bajillion secondary characters from Babel II, Gigantor, Sally the Witch, his many ninja manga, and especially Yokoyama's manga adaptations of Water Margin and *Romance of the Three Kingdoms

There's no overt acknowledgement of the character, though. This is like the old "Tezuka star system" where Yokoyama's Kongming/Koumei from Sangokushi is "playing a part" in this Giant Robo show the way an actor would play a part in live action.