Darryl and I have skirted around this topic a couple of times and never come to a definitive answer as to the wheres and whys of Giantish origins on Garnia world. We have also done a little wrestling with Dragonish origins too, but that can wait for another post. I spent a little time thinking about this today and came up with the idea that maybe Giants are from a plane/planet/sphere/whatever called Jotunheim; all of them, Good ones, Evil ones, smart ones, dumb ones, two-headed ones, et cetera, probably Ogres too, since they seem to be just a runty, stupid, Evil race of Giants. I decided on Jotunheim because, hey, maybe the Norse were right, eh? I mean this is pretty much Alfheim, right?
Now the thing about Giants in AD&D is, and remember Garnia was originally designed as an AD&D game world, that they are pretty much attached to some natural feature or elemental force, so I guess that means that Jotunheim must be a pretty elementally connected plane of existence. There are six types of Giant in the 1st edition AD&D Monster Manual- Cloud, Fire, Frost, Hill, Stone and Storm. Each type of Giant has some pretty amazing abilities, even the weakest, the Hill Giant, can hurl boulders up to 200' for 2-16 points of damage, and they come in groups of 1-10, larger groups (more than 4) have women and children and have a 50% chance of having Dire Wolves, Giant Lizards or Ogres as Guards for their lairs. Tougher Giants get better special abilities, immunities to, or lessened damage from certain types of attacks, better guards that are generally more suited to their elemental type, and seem to have a more magical, elemental connection.
So that's the where they came from anyway. Why are they here? At a guess I would say that the Good aligned Giants were happy to trade/communicate/cooperate with the Sidhe when the Sidhe more or less invented planar travel. That said, it doesn't have to be a magic belonging only to the Sidhe either; there's no reason why magically powerful Giants could not have invented gates too or at least used spells for planar travel. Aren't Titans just a type of Giant? They are really smart, use both Arcane and Divine magic, and Psionically* talented too. Anyway, maybe they came here for company, maybe they came to join the fight on either side once it got underway. They're here, that's the important thing; they're none too common in any variety, but they do exist.
*I dislike Psionics, but there's no reason anyone else running a campaign in Garnia couldn't use them. I never specifically disallowed them in the old days, they were just really hard to get and most people never thought to try.