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.
Haru Nakajima,the man who played Many of the toho monsters including Godzilla in the first twelve movies has said gaira the evil gargantua was his favorite to play due to the flexibility of the costume and you can see his eyes through the mask
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.