Just because GFs were only recently discovered by Dr. Odine doesn't mean that they haven't existed for who knows how long. Dr. Odine's research just probably led him to discover a way that GFs can be junctioned or what not.

Quote Originally Posted by Unknowns
But like in the game, you can draw GF from monsters, so GF might be something inside monsters in FF VIII all along?
Quote Originally Posted by BG-57
The GF's can't be intergral to the monsters they reside in since each major drawable GF resides into two different monsters (one of each in Ultimecia's time).
Well, outside of Ultimecia's Castle, there are only three GFs that you draw from monsters. Ultimecia's Castle is weird and time compression is going on so who knows how those monsters got their respective GFs, but the others might be explainable. Carbuncle and Siren are both rather weak GFs, relying on status effects for their summons rather than much of any power. So it's possible that Elvoret and Iguions somehow junctioned (or maybe ingested) these GFs. You get Eden in the Deep Sea Research Center, research=anything's possible, the researchers could have put Eden inside Ultima Weapon themselves, who knows. Also to note, every human/shumi that you can draw a GF from is somehow connected to Balamb Garden:

Leviathan from NORG
Pandemonia from Fujin
Alexander from Edea

so all of those GFs are more than likely juntioned to those people. And every other GF you get you have to fight, and then they agree to join you. I would conclude that GFs are simply monsters, but they have some intelligence that would allow them to fight with you instead of the innate hostility that all other monsters have against humans. I don't really know beyond that.