^ That sounds cool, but incorrect. Did he say Shigeru or Takayoshi?
Takayoshi Mizuki designed Gigan. He was working at a Weekly Shonen Magazine by Kodansha doing full color illustrations (during the monster boom). Kodansha recommended him to Toho when Toho was looking for an illustrator to make a "more colorful" monster (he said this was due to the popularity of Ultraman). He was told it needed to be a "monster of the universe" with no additional information about the script, and he was excited to be able to create his monster because professionally he had only been drawing other peoples designs. The recent Toho Champion Matsuri Perfection book from Dengeki Hobby Books has an interview with him where spells out where each design element came from and cites Toru Narita's design work as an inspiration.
http://www.art-micbox.com