So I've been doing some digging around on the Japanese internet, and I've discovered that Godzilland actually originated as a merchandise line from the 1980's, and the TV show was more of just an adaptation of it. But even more interesting, a book called "ゴジラ・ガメラ大怪獣," or Godzilla Gamera Daikaiju, was published in 1984, and features illustrations of both chibi Godzilla and Gamera kaiju done by the Godzilland artist. This means that there was in fact a piece of media featuring both Godzilla and Gamera together.