The immediate sequel to the original Godzilla, Godzilla Raids Again, usually isn't deemed to be a worthy sequel to the original. While it may not have the potency and pathos of the original, I'd argue there are some moments present in the film that attempt to reach the same heights of the original. It's arguable how successful that was but it's at the very least worth checking checking out.
The film's American version on the other had is a far different story. It re-contextualizes the film into the standard trappings of a run of the mill American monster movie. For all that can be said about the Raymond Burr version of the original Godzilla, it was by far the most respectful of the early Americanization that many Japanese monster films experienced. The film's use of stock footage that pales in comparison to the original effects and the infusion of stereotypical imagery makes for a much more dull and dry experience. And if one is not already a fan of the original Raids Again, then Gigantis does it no favors. What Gigantis, the Fire Monster attempted to do was to reduce Godzilla to just a generic monster, and egregious mistake to do with a character who is by far the opposite.