First of all, sometimes when you put on an item you will get an echo that indicates that Gates is being put to use. I never thought to try and figure out if those echoes came at certain threshold (-100AC; -150AC) or if it only came from an item that bumped your AC a certain degree (likely more than -10AC.)

However, if I remember correctly, if you are completely naked and put on a decent piece of gear you will get the echo, meaning that it likely doesn't 'kick in,' but rather is a sliding scale that starts as soon as you're wearing any single significantly hefty piece of gear.

Only an IMM could say whether it has a cap or not, but the fact is that there is a hard limit to just how low you can get your AC with gear. After all, you only have so many slots. From what I could tell, though, it scales more steeply the lower you go (ie. -300 to -400 is better DR than -200 to -300.) By the time I got to -400 (though I also had resist and inherent stoneskin) I was seeing absolutely absurd damage reduction. I remember in one fight against the Archmage of the HTOS, my group was taking ***DEMOS*** and I was taking decimates.

Good luck with this awesome legacy.