Godzilla vs Gigan (or as I saw it uncut (except the credits) in the theater, Godzilla on Monster Island) has never been a movie I cared for very much. released in the US 7 years after it was made (1978) I was past the gee whiz stage of loving every aspect of giant monsters.
I'd see Gigan in Famous Monsters #114 though there was scant information there on the movie. he looks cool enough. but after finally seeing it, I was quite disappointed.
I recognized every piece of stock footage in the movie, even more than there was in Godzilla vs Megalon (1973). seeing Mothra make an uncredited cameo screamed of sloppy editing, even to my 13 or 14 year old mind. it was really cool seeing parts of Ghidrah, Hedorah, War of the Gargantuas and Destroy All Monsters on the big screen but enough was enough.
I had read about Godzilla and Angilas talking to each other and remember discussing it with friends whether or not they would speak in English or Japanese. they spoke in distorted English, which was pretty cool. in the Japanese version they get words balloons, just like in a comic book.
it's amazing the difference in thought processes even a few years can make. when I saw Megalon in the theater in 1976, I accepted everything it presented and never once thought about why Seatopia would send Megalon to destroy Japan, a nation who has no nuclear capability. but with Gigan I saw every fault. it just tried too hard to be an Ishiro Honda movie and failed.
nowadays I like the movie a bit more, knowing how small the budget was for these 1970s flicks.
this is atomic breath, not a damn beam weapon from his mouth!