Well the relationships of the Kaiju sometimes change with every other movie. So while Godzilla was fighting Anguirus the first time they met (in the second Godzilla movie that is), later they became allies. As in this one. And it is necessary they team and tag together, because apart from the new (mentioned) Gigan in the title, we also get King Ghidorah as an enemy of earth and humans - which means enemy of Godzilla too of course.
And while there was always communication between the Kaiju, mostly through their screams and cries, this time we get text bubbles with what they mean. Interesting choice and maybe related to the fact they tried to go kid friendly at the time. Also a nod to comics/manga in general? Very likely.
The monster on monster action (get your mind out of the gutter if it is there) is really something to behold - and fun to watch. Men in rubber suits wrestling other men in rubber suits. While humanity looks and hopes/roots for our beloved Godzilla. Obviously there is a lot of History here. Movie history that is. Something you can have knowledge of, but you don't need too. It just gives you more appreciation of what is happening on screen - and might make you understand why effects that seem dated now, where all the rage (no pun intended) back then.
Enjoy if you can, because this is quite the fun ride