It's not often that a once-in-a-lifetime item comes along, and you have the opportunity to add it to your collection. I ran into an Ebay seller who had a connection to the defunct Trendmasters company, and he was selling many interesting items, including Godzilla action figure blueprints and design art, some Doom Island toys, and a handful of prototypes, as well as many non-Godzilla Trendmasters items.
Besides saying that his items were acquired from the Trendmasters offices, and were supposed to be destroyed, he told me that he believed this prototype Gigan was used at a Toy Fair to promote the second version of the 40th Anniversary Boxed Set that we've talked about here before, only here with a mock-up set that used hand-painted figures. This would have been around 1994.
The boxed sets included non-posable, hard rubber kaiju that were each about 4" tall. You can see from the photos how the paint scheme is different than the finished product! The prototype also has a production number written in permanent marker on the bottom of one foot.