This makes sense, as the actual book I have that includes "Gamera vs Leoman" (1971) was published in 1975. The author, Jeff Rovin, probably used the article August mentioned as his source.That [Leoman] was originally reported in an article about Daiei fantasy films in an early 1970s issue of Cinefantastique. There were a lot of titles thrown around for almost all the films, some with "Gamera vs. (insert name) Monster (insert name)" on their covers -- before final names were decided upon, monster names were always in development, and tossed around, before they registered the final moniker to be used (the monster slated for that film was eventually named "Garasharp").
I just looked this up, and the screenplay in question was titled "Gamera vs. Twin Monster W" (Gamera tai Futari Kaiju W), which was a placeholder title. I have never seen anything official concerning that "Leoman" title, sans the aforementioned Cinefantastique retrospective (although it may have originated from one of the many names that would be tossed around for Gamera's adversaries).
So, even if it was a placeholder monster name, and nothing else, I'd still like to see it used on a future Gamera film, just for posterity sake .