Man I remember GoW back in the day. My buddy was obsessed with it, he'd always host multiplayer matches online and just annihilate everyone.
I'm not sure if GoW was the first game with a horde mode, but I do remember it being particularly amazing and it was definitely my favorite part of the game. Which Gears is your favorite?
I think 2 is my favourite. I put a stupid amount of time into 2, both campaign, horde, and general multiplayer. Overall, I'm a huge fan of the franchise.
So much so that I've put myself through the hell of finishing them all on Insane difficulty. ;^]