the trick is to keep breathing

Pretty much, Garnet actually came from a race of summoners, at least some of which are born containing eidolons (or I think, maybe since she was being whisked away to safety they imbued eidolons into her at a young age). These eidolons can be contained in gems, or come from them, both ways work really. I don't quite know exactly where they come from, but then same goes for most cases of FF summons. FF7's summons come from materia for example, which you have to take a logic leap of faith to assume they're a life force within the planet taking residence in materia.

I do strongly suggest replaying it. If for absolutely no other reason than the chocobo minigames. I have a feeling you didn't get to fully experience them. :P There's nothing quite like riding a giant horsebird out into the ocean and pecking at water until a treasure chest comes out of the bubbles and shoots rape gas in your face that sends your horsebird to the gay bar of its dreams. Seriously though, the hot and cold digging game is tons of fun and addicting, followed by exploring areas to get treasure from the chocobo pieces you find.

I'm sure you'll find things to write about it once you play it again, too. I myself had some fun with Garnet and her delightful "I know, I'll stow away in a bag of pickles to get past this gate!" event.