Hi, I too have often looked for "Gamera 4: Truth" and never found it. But back in 2005 while I was working on my "SFX GIANT LEGEND LINE" article for my Classic Henshin Site, I did some looking around in regards to the film.
LINE is a series produced by Rapid Progress, the company that produced "Gamera 4: Truth".
On my LINE site I wrote this in regards to the Red Kong episode:
"This episode guest starred Hotaru Yukijiro, who played inspector Osako in all 4 Heisei GAMERA films (the fourth one being a fan film). Here he appears to be playing the same character! There is some connection between LINE and the fan film GAMERA 4:TRUTH. LINE was produced by Rapid Progress, responsible for ASSAULTMAN. One of the monsters in ASSAULTMAN is the very slightly modified Gamera suit used in G4T. Red Kong, by the way, is a giant angry gorilla who is addicted to bellydancing. Unfortunately when he dances he destroys the whole town. He is NOT red. His eyeballs are white, just like Godzilla's in the Godzilla movie directed by Gamera trilogy director Kaneko."
If you look at LINE you can see what Rapid Progress's output looks like:
http://www.arislan.net/henshin/raine/index.html