What is this giant robot anime?
You're probably looking for Getter Robo G. There's several TV series and movies based on this iteration of Getter. The movies are all cross-overs with other Go Nagai mecha franchises.


Getter Robo G
Great Mazinger tai Getter Robo G - Kuuchuu Dai-Gekitotsu
Grendizer - Getter Robo G - Great Mazinger Kessen! Daikaijuu


All from the 70's.