Canon

[GoktimusPrime]

Why wouldn't a planetoid with enough gravity to keep objects (and its inhabitants) from drifting off the surface, not have the gravity to attract and keep gasses or liquids? The lightest element (Hydrogen) is throughout the Universe, and this planet was mostly metal (or rock, depending on your source), so where's the impossibility that elements (Oxygen, Nitrogen) in between them could not exist?

A fair bit of the matter on Earth came from extra-terrestrial debris over the millions of years, including during its formation with the rest of the Solar System... why couldn't Cybertron have formed with, or attracted, these elements?

Not impossible, but what an incredible FLUKE that an alien world with NO biomatter or biosphere (or even soil) can sustain an Earthlike atmosphere! What are the odds?!?

Unless a writer tried to explain its existence, or tried to explain how it couldn't have existed in the first place, I don't see how or why some fans make such a big deal about it,

Cos it's fun for us nerds! <snorting.laff> That's why I enjoy reading books like this which basically takes the Star Wars universe and tears it to shreds with science! It doesn't mean I've liking Star Wars, but I still find it an interesting read.

When we watch or read fantasy and scifi we often suspend disbelief of high improbable or impossible things; but every once in a while I find it interesting to stop and think about what would be possible and what wouldn't. As Kevin Smith once said, if you ever had a rocket pack you'd "burn your ass off."

or even claim that Cybertron shouldn't have an atmosphere or liquids at any time.

Not just any liquids... but water. And as Sky Shadow rightly pointed out, canonical evidence from G1 indicates that water doesn't exist on Cybertron. And yeah, that waterfall in BM could be some other kind of liquid. HCl perhaps.

Think we could say humans abiltiy to survive on Cybertron boils down to plot device meant for children.

"A wizard did it."

Maybe Cybetron is smaller, but because of the denser materials it's made from, it has the same or similar gravity to Earth..?

This is true, although it would have to be extremely dense because Cybertron has a LOT of subterranean cavities!