I have no Getter Robo history, I see there was a TV series in the 70s and some OAV in 2000s. Would you need to watch the TV show, OAVs or both for this?
All animated Getter adaptations are completely standalone and none of them adapt the manga directly so far. Meanwhile, the main manga series are all connected
Getter Robo -> Getter Robo G -> Getter Robo Go -> Shin Getter -> Getter Arc (unfinished due to Ishikawa's death)
Getter Go was written more than a decade after Getter and G, so there's a big change of tone and focus there, and the rest of the manga follows that (there are also some new chapters added in collections that attempt to make the first two series line up better with the later ones, when you're reading it the art style change is obvious). Shin Getter is a prequel to Go.
As far as animation goes, there are the 70s tv series (Getter and Getter G) which are connected to each other and were very loose adaptations of the Getter Robo and Getter Robo G manga, changed to fit the mold of "weekly super robot shows for children" that they had at the time. Then there was a standalone Getter Go in the early 90s, which was another loose adaptation of the Getter Robo Go manga, but, on top of that also eliminating the connections to previous series (which eventually pretty much take over the plot of the manga).
After that you get the OVAs - first there was Getter Robo Armageddon. It's a completely standalone original story using elements from many different points of the manga, but the big selling point at the time is that it was the first time they were pushing something closer to Ishikawa's actual style in animation. It features the cast of Getter G and some of Go (although these ones pretty much in-name only) while featuring many elements from Getter Robo Go (it talks about a backstory regarding lunar wars and an accident which might make it look like it's a sequel to something, but all that is original to this anime).
Then there was Neo Getter vs Shin Getter, a very short and episodic OVA which mixed some early chapters from Go (introducing the new team) with the later ones (where the plot becomes connected to the original series). Yet again, it was completely unrelated to any existing adaptation though, aside from using a character and robot that originated from the 70s tv anime rather than the manga.
There's also "New Getter Robo", which starts as a direct adaptation of the original manga, but changing the enemies from dinosaurs to Oni. However, it then introduces a new original third pilot and starts using unadapted material from later manga like "Shin Getter".
