Only really powergamey ones would even consider it.

Also, you neglect to mention that by rolling duergar, you lose out on some of the best swords in the game, and have a much more limited selection of cursed weapons you can use. Its a lot easier to find the best two-handed swords than it is to find the best one-handed swords. Equipment is critical for build optimization, in my opinion.

Duergar have slightly better stats than fire giants, but a much worse vuln selection.

I'd take cry of thunder over gates anyday, haste by default increases your damage, adding more attacks and better defenses, and the healing from the warcry is hella useful too.

Finally, if you're running gates, you're very limited in your armor choices. You already have a lot to gear for - you want +str/+dex, damroll, and a bunch of other stuff. Adding a/c just seems to me like you'd be stretching your optimal set even thinner and more patchwork. Furthermore, if you're planning on running dern armor, that basically kills off your stat edge on duergar, as dern armor is like -20 dex for the whole set. Any fire giant will have better strength AND better dex than your duergar wearing dern armor.

Sorry to crush your dreams, but really, your build is all screwed up.