Although no one would confuse Godzilla vs. Gigan with a Great Film, it has a kooky appeal and moments of visual flair that make a positive contribution to the G-universe. One obvious example would be the introduction of the space monster Gigan. He looks mean as all get-out; he sounds chillingly robotic; he's got the belly-saw going; he's a force to be reckoned with! When Gigan's profile is illuminated by refinery fires late in the film, the message is sent that Godzilla is facing a formidable foe. Not until Final Wars would Gigan actually be destroyed--and even then, at his own hand (or should I say flying saw-disc). I also like the quirky cast, and it is worth noting that Godzilla was getting it handed to him by the Godzilla Tower laser cannon until the humans blew it up! I'm of the opinion that the Godzilla/Anguirus duo is fun to watch, and their gang-up on King Ghidorah is pure kaiju teamwork--Anguirus making the most of his spiky anatomy, and Godzilla working the power slam.
It is only fair and right to point out that Godzilla vs. Gigan does suffer from "recycling" that was going on at Toho in those days, and it is disheartening to see poor-quality stock footage inserted into the battle and rampage scenes. My six-year-old son impressed me mightily by identifying battle footage from Destroy All Monsters, and because of the inserts, the Godzilla suit obviously changes at least three times. The thing is, the movie would have been fine without the filler. I don't know if they were trying to hit a given running time or what--but from my point of view, the inclusion of the stock scenes was simply unnecessary. The savvy listener will similarly be able to pick up on the recycled nature of the musical score. Those who wish to build a case against Gigan admittededly have plenty of material with which to work, especially if they want to get into the whole "talking monsters" thing. (The Japanese version, with the cartoon thought bubbles, at least related to the comic book theme...oh, never mind.)
So, yes, Godzilla vs. Gigan has some glaring flaws, and if there is something in you that resists the idea of space monsters doing a synchronized loop-de-loop on their way to destroying the Earth, you should probably stay far away from this movie. But that "anything can happen" spirit fed my early G-fandom, and I'm still more than willing to go along for the ride.