The film's title should be written in English as "Gamera, the Guardian of the Universe." The phrase "The Guardian of the Universe" [yes, there is a "the" in the title on both the Japanese and U.S. title cards] is a description of Gamera. Colons are for subtitles. The title isn't saying this is the Gamera franchise about a new "guardian of the universe." It's calling Gamera the guardian of the universe.
The film's title should be written in English as "Gamera, the Guardian of the Universe." The phrase "The Guardian of the Universe" [yes, there is a "the" in the title on both the Japanese and U.S. title cards] is a description of Gamera. Colons are for subtitles. The title isn't saying this is the Gamera franchise about a new "guardian of the universe." It's calling Gamera the guardian of the universe.