GCA are the mongols. Keshiks ignore terrain penalties, are stronger than horse archers and ignore 1st strikes. Spearmen will be the best counter (we'll need hunting).
I'm a little behind on my demographics stuff, but last I checked they didn't even have animal husbandry yet (can't see horses). I'll look into it tonight and see what I can find out.
EDIT: They may have gotten it in 3000 BC.