DVD Commentary: The DVD release by Classic Media includes one by Steve Ryfle and a few others. It plays over and talks mainly about the American version, but it also talks about the Japanese version as well and how the movie in general compares to the original.
To elaborate: He's the reason why the Gigantis film became what it was because he said the renamed Godzilla to Gigantis was to avoid confusion. He even said the original title was "Angurius" when the original name is Gojira no Gyakushu (Godzilla Strikes Back). This blatant attempt to pretend this is not a Godzilla sequel made it laughable to the real film of the franchise.
On Toho's side, Iwa Omori, the head of Toho Studios, told Tomoyuki Tanaka to make another Godzilla film after the success of Godzilla. Depending who you ask, it was better or worst direct sequel.
Old Shame: Hiroshi Koizumi lamented that upon watching movies he appears in, including this film, that he felt he hasn't put enough potential in the movies he appears in.
Recycled Script: It's basically Godzilla 1954 with another monster added in. However, since Ishiro Honda was not involved in this film (as Honda directed a love story afterwards), Shigeru Koyama wrote the script with no alterations from Motoyoshi Oda. However, without a means to defeat Godzilla without Daisuke Serizawa, the Japanese are screwed.
What Could Have Been: Interestingly enough, Godzilla Raids Again wasn't intended to be merely dubbed and distributed. It was going to be called The Volcano Monsters where Godzilla would be just a Tyrannosaurus rex and Angurirus to be an ankylosaurus (how they are gonna explain a T-Rex that clearly looks like Godzilla is beyond us). Godzilla (or rather the T.rex), was to fight the ankylosaurus and move to the North Pole to lay its eggs. While Toho decided to help by sending new suits and made a catalog of the production, the American company went bankrupt and the suits disappeared. So now, The Volcano Monsters was ultimately cancelled. This explains why it took a whole 3 years on why it wasn't released in form. Eventually, the film ended up being called, by the massive face-palming of Steve Ryfle (and his fellow commentators) and fans everywhere, ''Gigantis The Fire Monster".