Continuities
The TV series, OVAs and manga are considered to be three different series because of the various differences between them.
The TV series Getter Robo and Getter Robo G are Toei's version of Getter Robo based on Go Nagai and Ken Ishikawa's Getter Robo concept and not the manga. Getter Robo Go is a futuristic Getter Robo show that mimics the old Getter Robo TV series in style (Family friendly). Like the previous two, it also follows the monster of the week formula.
The OVAs are mostly based on the Getter Robo manga, and sometimes other Ken Ishikawa manga like Kyomu Senki. Each OVA is set in a different alternate continuity and thus can be watched in any order.
The Getter Robo Saga manga compilation by Ken Ishikawa (Getter Robo, Getter Robo G, Getter Robo Go, Shin Getter Robo, Getter Robo Ah) is considered to be the main continuity of the franchise as each entry is connected to each other. The manga not by Ken Ishikawa, like Go Nagai's Devilman vs Getter Robo and Getter Robo Darkness, aren't considered in the Ken Ishikawa continuity. The only exception is Getter Robo Hien by Naoto Tsushima which is apparently based on the notes left by Ken Ishikawa after his death.
Read more about this topic: Shin Getter Robo