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So, as you may have noticed by the "Tech Levels" essay, I've given some thought to the ways in which the Godzilla and Gamera movies are fundamentally different, and I've come up with another cluster of related differences. To wit, Gamera is almost always about the one on one duel, while Godzilla movies are often multi-kaiju throwdowns, brawls and tagteaming.
Consider Showa Gamera: with two exceptions, there's only ever Gamera and zero or one other Kaiju involved. The exceptions are Gamera vs. Guiron, in which Guiron chops up a Space Gaos, and the clip show that is Gamera, Super Monster. And even in those cases, the battles are always one on one. Gamera vs. a single enemy Kaiju, or two enemy Kaiju against each other.
Heisei Gamera has a bit more ganging up, but it's kept within "type". Gamera vs. a bunch of Gyaosim (that's not the official plural of Gyaos, I just like saying "Gyaosim" for some reason), Gamera vs. Legion and Jim Henson's Legion Babies. Millennium Gamera also features a flashback to Gamera vs. Gyaos swarm.
On the other hand, Godzilla's constantly teaming up with others (Anguirus, King Seesar, Rodan, sometimes various mecha-kaiju, lots of Mothras) and getting double-teamed (Ghidorah and Gigan, Megalon and Gigan, Kiryu and Mothra, etc). And sometimes there's out and out brawls with four or more Kaiju on the field at once.
So...what's at the root of the differences?
First, a lot of it has to come from the simple fact that Toho has a larger roster, exacerbated by the fact that Gamera's foes rarely make it out of their intro movie alive. The only recurring Kaiju other than Gamera himself is Gaos/Gyaos, and that's only because it's a species rather than an individual (I'm leaving out Gamera, Super Monster in this case, since it's really just a clip show, and takes place in an alternate reality where the other Showa movies are fictional). Godzilla is still pretty lethal, but he simply leaves a lot more of his enemies alive, and has more of them to begin with. Thus, there's more hanging around waiting to double-team him, and more available to change sides and become allies.
Second, the roles of Gamera and Godzilla diverge somewhat after their original appearances as Angry Forces of Perverted Nature. Gamera quickly settles into the role of defender, a sort of Kaiju cop or Captain America who stands between the children of the world and total annihilation. Godzilla, on the other hand, is just this guy, y'know? Sometimes he helps, sometimes he just wants to stomp stuff...if he can't be arsed to come deal with Ghidorah or whoever's on a rampage, someone else needs to be around to hold the line. If Godzilla's being a jerk today, at least a Mothra larva or two can be trusted to hold things together until Godzilla's in the mood. Thus, there's no real need for other "good" Kaiju in the Gamera setting, since he'll always come to save the day...and if he gets injured, the squishies can usually keep it together while he recovers (i.e. the kids in the minisub, the army making it rain on Barugon, the army blowing up the Legion Babies, etc). Not to mention, in Heisei they make Gamera a final weapon intended to clean up after previous weapons, which is as good as saying "everyone else is an evil monster, it's Gamera versus the world in a last ditch effort".
Third, Godzilla is KING of the monsters, baby. Sometimes you just need to pile it on in order for there to be a plausible threat. Gamera is generally not as powerful (visible-from-orbit final move in Gamera vs. Legion notwithstanding), so it doesn't take a kaijupile to pose a significant danger to him.
Fourth, recurring threats are less threatening threats by their very nature. When Ghidorah comes back, you know Godzilla can beat him (since he did last time), so Ghidorah either needs an upgrade of some sort, or a team-up. A repeat of the original duel is out of the question. Gamera foes tend not to come back at all, making it less of an issue. In Heisei, you could consider Iris to be an upgraded Gyaos, and in Gamera the Brave they up the ante by throwing a Gyaos swarm at him, but that's pretty much it for recurring threats to Gamera.
So, to sum up, Gamera's a workaholic with a small roster of mostly non-recurring threats that tend to be tough enough to threaten him on their own. Godzilla's as much threat as help in a world chock full of kaiju with widely varying motives, many of whom have been proven unable to defeat Godzilla on their own. And that's why Gamera's all about the duel while Godzilla's as likely as not to go for brawls.