#gaira

Starting off, the explicitly canon female kaiju of the Godzilla cast:

Mothra

Biollante

Zilla (I think the movie tried to deny it, but the cartoon confirmed it, although final wars zilla is another question)

Megaguirus

Monsterverse actually brings a number of explicitly female kaiju into the fold including:

The female Muto from 2k14

Barb, the other female Muto

Scylla

Tiamat

Na Kika (formerly Kraken)

Shimo

Phosphera

Some of the other unseen titans from King of the Monsters might also be ladies, but given as they are still unseen, that's just blind speculation

Now we get to the interesting part, the fan interpretations

Manda, Kumonga, and Destoroyah all being female is a pretty common and old fan interpretation, I know from firsthand experience that it at least dates back to the 2000s, but it could even be older

Baragon by comparison is far more recent, and I know for a fact this interpretation can be sourced primarily to the fact that in GMK, Baragon's performer was Rie Ota, and a behind the scenes clip of her doing the roars in the suit made waves around the fandom and even outside it in the 2010s

Hedorah is another more recent one, but interestingly enough I've also seen enby, bigender, and genderfluid interpretations of Hedorah as well. I guess there's something about the smog monster that screams gender to fans

Titanosaurus is again a newer interpretation, but fascinatingly a even newer idea has popped up in the past couple years that Titanosaurus is the kaiju equivalent of a femboy twink

Rodan being female is a very interesting case, both in origin and mainly due to the fact it only really applies to the Showa continuity and various fan continuities that desire. Basically the origin of this is that in the original 1956 film Rodan, there are two Rodans, a mating pair. At the end they both fall into Mt. Aso. When Rodan reemerges from Mt. Aso in Ghidorah the Three-Headed Monster, there's only one. There's been speculation for years as to the origin of this Rodan, but for the longest time it was either assumed to be the male or somehow the child of the pair from the original movie. But in 2010s a new idea rose to greater prominence: What if the Rodan that survived and subsequently appeared in the rest of the Showa films was the female of the pair?

Battra is an uncommon interpretation, but I have observed it

@lydiathespiderqueen did some fanart of a female version of Toho's Frankenstein, as well as the Gargantuas, and it's really good

and @thesilkenlair did a compelling analysis of Spacegodzilla being female due to possessing physical characteristics of Biollante, as well as their behavior in the movie lining up disturbingly well with a phenomenon in nature where an animal will kill or try to kill the extant children of a prospective mate so undivided attention can be given to it and any potential offspring it may have