Why which, exactly? Your question is a tad too broad for me at this time of night. ;-)
> Are the gates holes in the crystal sky dome or in the earth
> connected to the Underworld or what is going on ?
Middle option. I think the usual "model" is much as the Gates of Dusk are described in KoS (big gates, lead to underworld). They ought not to be holes in the Sky, since then they'd be visible, and lead to the aether, not the underworld (in the Generic Cosmological Model, at any rate). What they actually look like, and precisely in what the "hole" is "in", is getting into more questionable areas...
> I'm interested in both the mundane and the mythical reasons for why the
> Palaces of Dusk and Dawn are sliding around like eggs on a greasy frypan ?
Nonono, not those. The Sunpath planets (and the Sun (well, du-uh)) all rise directly in the east, and set Dead West. Only the Southpath planets, to wit Tolat, the Twinstars, and Artia bother with the Eastern Mouth and the Dodging Gate, the variously-lubricated ovoids of the above metaphor.
The Mythic Significance of said motion is presumably that these are the "unreliable", and least "Solar" of the planets (and hence most "Lunar"?(?)). For a more mundane justification, note some of the wacky things RW planets do in the Sky. Not only not they not rise and set in consistent places reliably, but some of them exhibit retrograde motion, precession of their orbits, and they all have annoyingly fractional (and titanic lcm) periods.
In fact, Greg/Steve's proposal makes the supposedly ultra-mysterious and zany Southpath far more predictable and regular than the earthly sky. If only one group of celestiologists on Glorantha have managed to predict something that essentially repeats all the spontenaity it's ever going to exhibit inside of 27 years, then the others must be fairly spectacularly dense. (Conceivably the Western Astrolomers have an Aristotlean distaste for much in the way of practical observation, while the Kralorelan Sages of Heaven have been reduced to congenital stupidity by successive waves of Stalinist purges when they (inevitably, and recursively) get their predictions wrong.)
Slainte,
Alex.
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