I didn't know about the gate giant until I sniped him from that dead-end outside the cage lift. I noticed he and the firebomb giant drop more souls than the stone-setting giant, by about 40%. I guess it's because they don't respawn.
Does he drop anything good? Because I didn't know I could reach that balcony until warping out, doing some other nonsense somewhere else, then coming back to fight the titanite Demons, and there was nothing up there.