God Mazinger is a manga, anime, and novel series by Go Nagai in 1984. Despite the title Mazinger being present, this spin-off series has no connection to previous series. Instead it is an original story about a young man who ends up in an alternate reality.
While God Mazinger has no connection to the other Mazinger series, it is indeed part of the Mazinger franchise as besides the title and the same author as the other series it is featured in the Mazinger Bible along with other manga such Mazinger (USA) and MazinSaga.
The anime version was originally going to feature 24 episodes and while material was drawn up the final epilogue episode was never shown. Part of the reason is because the air date was removed for the 1984 Olympics, and TMS for some reason decided to cut the epilogue from the show entirely.
(Source: Mazinger Wikia)