There is the manga, which is more or less treated as the main canon by Ken Ishikawa fully translated and readable from here! [link] The chronological order is Getter Robo, Getter Robo G, Shin Getter Robo, Getter Robo Go (produced before Shin) and Getter Robo Arc. (Mind you, Getter Robo Arc remains unfinished due to Ishikawa's unfortunate passing)
Animated wise, the classics Getter Robo and Getter Robo G I don't think are translated anywhere (Though you can watch an old 80s English dub of G as Starvengers, but eh). Getter Robo Go anime, PASS! The OVAs are good to watch separately, as they're all their own thing. Shin (Change) Getter Robo/Getter Robo Armageddon seems to be the favourite. Shin Getter Robo vs Neo Getter Robo is kind of an adaptation of the Go manga. New Getter Robo is kind of an adaptation of the original Getter Robo manga. But both differ so much so from the manga.
Game wise, play any Super Robot Wars game as they'll have a guaranteed Getter Robo appearance (with the exception of 2-3 handheld games).