During the process of dubbing the film for American audiences, Russ Tamblyn's audio track was lost, meaning he had to re'dub all his lines, which might explain why he sounds so disinterested throughout the movie.
Haruo Nakajima, the man who played many of the Toho monsters (including Godzilla in 11 of the original 15 movies) has said Gaira, the evil Gargantua was his favorite to play due to the flexibility of the costume and because his eyes were visible through the mask, allowing him to emote better.
Russ Tamblyn reportedly hated working on the movie and often disobeyed the film crew's orders, causing much frustration to his peers. He considered the original script dreadful and rewrote or improvised a lot of his dialogue, at times confounding his Japanese interpreter. Nevertheless, he has warmed up to the film decades later and now claims that he likes it.
The trivia item below may give away important plot points.
Even though both Sanda and Gaira died at the end of the film, there was talk of one or both of them returning to "star" in a film with Godzilla. Unfortunately, those plans never made it past the talking stages.