Ghidorah and Gigan are aliens so they don't follow Earth rules, especially in terms of biology. So, unlike the rest of the titans, who just have different versions of themselves that may as well be their own persons, the Ghidorahs are more or less actually related while the Gigans are kinda related in the same sense as the Earth Titans but in a different way.
Ghidorahs are parthenogenetic, so if they don't have a partner to impregnate or get impregnated by, they can asexually have spawn but it's either they spontaneously get pregnant or they chop more than one limb (because Ghidorahs are usually born as triplets) off and if it grows into another person, well they have a Ghidorah Jr.. So, it's either OG Ghidorah accidentally had kids via lost limbs or one of them just went "Meh, I feel like having spawn". Kaiser and Desghidorah are probably adopted tho.
Meanwhile, personally, I feel like, thanks to the experimentation and upgrades, I don't think Gigan can ever have kids. Like, if I make a killing machine, I'd make sure it wouldn't be able to create little hellspawn of their own. However, I would have some of his DNA somewhere in case he dies and I need a murder chicken. So yeah, Millenium!Gigan is OG!Gigan's clone, just modified and upgraded even more. It's similar to how the Earth Titans have different versions of themselves except, instead of a process like with cloning, the version just suddenly comes into existence. Oh, Gojira's getting popular in America? Well, now there's an American Gojira! Where did he come from? Who knows and who cares!
Also, also, another idea: Showa Ghidorah first coming to Earth and yeah, they come in as an asteroid but being ancient times, you know the people back then would be thinking "GODS!!!1!!1" and start worshipping them. So yeah, imagine going to an ant hill to stomp some ants except the ants think you're a god and start worshipping you. Yeah, that's their situation. One brother is amused and wants to play along, One brother is annoyed because 'WE ARE NOT THE MESSIAHS' and one brother just wants to destroy stuff. Basically, it be looking kinda like 'Life of Brian' except Brian is three brothers who wanna destroy everything and conquer shit.
Meanwhile, when they found out they were being mistaken for gods, the Gigans and Megalon if I add him are pulling a Road to El Dorado scheme in the bg.
Edit: better explanation hopefully