1. - 20 dex from dern? Wrong. -13 at max.
2. Dern gear activates Gates out of the box, other slots just add to it.
3. Dern armor grants you awesome save versus spell as well, out of the box.
4. Dern armor grants you awesome damage roll out of the box.
5. Dern armor affects "to hit" although, which may be an issue depending on your build.
6. Duergar equipped well is as good if not better than fire giant equipped well
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.
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.
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.

If you are going to post at least post the correct information.