Great Mazinger vs. Getter Robo is part of the Go Nagai crossover series. This is a short OVA, about 30 minutes in length, premised around the Tetsuya and Great Mazinger competing with the Getter Team to destroy alien invaders threatening Earth.
The two teams find themselves pitted against a space monster attacking Japan and a UFO ship that summons the monster. Essentially Tetsuya and the Getter Team have to learn to work together or face defeat at the hands of these alien threats.
The animation is very old school and has all of the goofy cliches that you would expect to see from the Mazinger and Getter Robo series, right down to calling out every attack and movement that the two robot suits make. I was a personal fan of the Boss Borot appearance. The comical humor really and sense of justice really made this a lighthearted watch.
I would recommend taking a look at this OVA if you have any interest in Mazinger or Getter Robo. My only caution is to get yourself a little familiar with both of these series or else it can be a little hard to follow. I ended up having to Wikipedia some things to get up to speed after watching.