Just to make it clear, Glorantha is not based on any earth cultures, it is a setting all by itself.
Having said that, there are some things that can help understand Gloranthan cultures a little better. They might also make you think about Gloranthan cultures differently and might lead to some very wrong conclusions, as Gloranthan cultures are different to real world analogues.
Let's start with the easiest ones:
Pentians are loosely based on Mongols, or rather Turkic steppe nomads. If you read up on the Mongols or other Turkic steppe-nomads then you will get ideas about Pentians.
Praxians are based on a cross between the steppe nomads and Native American Plains Cultures. Read up on Yak riders, Reindeer Riders, Tibetan nomads and stepe nomads for ideas about the tribes and culture, also any Native American Plains Mythology for ideas about the mythology of Praxians.
Heortlings loosely based on a mixture of Heroic Cultures, Celtic, Anglo-Saxon and Viking, so read up on those for ideas about the clan/tribe culture. Their mythology is not so similar, so reading up on germanic/Norse/Celtic mythology won't help that much.
Now the harder ones:
Dara Happa is very, very loosely based on the Bronze Age cultures of the Fertile Crescent, large rivers, cities with ziggurats and god-emperors. So, reading about those might help with some of the flavour.
The Lunar Empire is very, very loosely based on later Fertile Crescent cultures, I think of them as being almost Assyrian/Babylonian, with Ishtar being the base of the Red Goddess, but that is a very loose comparison.
Kralorela is loosely based on Cathay, Ancient China, so its mythology/culture might help
Vithela is loosely Nippon, so reading up on Japanese medieval culture might help a bit
Pamaltela is loosely based on Australian Aborigines, so their mythology might help a bit
Now for the others:
The West used to be based on medieval Christianity/Islam, but really isn't
The Holy Country has really nothing to do with Ancient Egypt, despite their king being the Pharaoh
Glorantha takes its ideas from all sorts of places, pulling bits in from here, bits from there and merging them with made up stuff that isn't based on anything in earth mythologies and cultures.
So, the Sky and Earth being forcibly separated comes from Egyptian mythology but that doesn;t mean that similar things come from Egyptian Mythology, for example.