Tohl Narita: “As its name suggests, this is a composite of a gamagaeru (frog) and kujira (whale). I remember Mr. Takayama had a hard time to plant beads all over the body.”
According to the sculpting plan of Gamakugira/Gamakujira drawn by Ryosaku Takayama based on the design done by Narita, the beads are described as vinyl beads.
The beads seem to have included the plastic ones from toy necklaces, and I feel like I read somewhere that the materials were obtained by Mrs. Takayama as her husband was extremely busy in sculpting kaijus one after another while I believe she should also have contributed a lot to the making of kaikus her husband devotedly worked on.
Although the monster appeared as Gamakugira in the episode script written by Mamoru Sasaki before Narita designed it accordingly, it seems to have been a name Sasaki provisionally gave to the monster.
It is likely that Sasaki simply put the words representing “frog” and “whale” together expecting Tsuburaya Productions would name the monster properly afterwards, so he seems to have been surprised to find the provisional name he gave to the monster in quite a haphazard way was actually used in the show.
While this episode was directed by Akio Jissoji with the script written by Sasaki as the two allegedly worked in perfect tune with each other all the time, it is widely known among fans Jissoji was disappointed at Gamakugira when he found it on the water set as it appeared to be such a laid-back kaiju like a good-natured dude bathing in a hot spring at leisure.