2. Dern gear activates Gates out of the box, other slots just add to it.
Sure, if you consider gates worthwhile on a sword spec. There are still better choices.
6. Duergar equipped well is as good if not better than fire giant equipped well
I agree, I just don't consider a derns duergar to be equipped well. Dern is an easy regear source for duergar/dwarves, not the optimal equipment. If you're planning on building a gates warrior around dern armor, at least pick a spec that doesn't get heavily penalized from the dex penalty.
7. The only possible point you may have is the lack of dex malediction depth a duergar would have compared to a fire giant. But even then, the 7 different dex almost makes up for that.
9. The equipment you would be wearing as a fire giant to cover stat loss you would be for a duergar anyway, not sure where you came up with that pile about fire giants having better dex.
10. The waaaay better dex plus your comparable strength makes the duergar a better tank in almost all ways than fire giant with swords
11. This is a build that is well known and very very successful. Not sure what you are talking about here about it being "screwed" up.
I know what Dern armor does.
If you're wearing several pieces of armor that add up to -13 dex, your duergar will have significantly less dex than the fire giant. If you're wearing full spiked, its a huge amount of dex loss that will also take up several slots of your equipment, forcing you to gear for dex loss in your other slots rather than more useful abilities. By comparison, the fire giant will wear any kind of armor.
Since the fire giant has 25 strength and the duergar has less, and the duergar will have -13 dex, bringing it under the fire giant, your base stats from this armor will mean that the duergar has less strength and less dex than the fire giant. If you gear to cover dex loss, that will take almost every other armor slot you have, rendering your bonuses complete shit. Also, gearing for dex coverage means gates will be less effective, as you will be forced to equip items which boost dex over items which boost a/c and other important stats.
8. Easier to equip duergar if stripped than fire giant due to duergar/dwarf only equipment versus universal equipment that a fire giant has to wear.
Partially true, but not entirely. There is a lot of very nice giant-only equipment too, including some of the best swords in the game.