I did a quick search on Google under "gamera guardian of the universe blu-ray" and turned up a page on the site Blu-Ray.com listing a Japanese Blu-Ray release for the Gamera trilogy with an Aug 2009 date:
http://www.blu-ray.com/movies/Gamera-90 ... -ray/5291/
The purchase button links out to Amazon Japan which looks to be the same set (different cover art though):
http://www.amazon.co.jp/gp/product/B0024DGNM0
Since KPA mentioned that Millcreek has been in contact with Kadokawa's US sales rep, I would imagine that a Blu-Ray release would use the same HD prints as the Japanese release.
Also of note, it seems Kadokawa has also released a Daimajin Blu-Ray set.
IF Millcreek has taken an interest in releasing Japanese films & TV series then Daimajin may well be on Millcreek's short list. Of course only time will tell.