GNB tanking is fun because of its rotation. Sure, it's no stormblood DRK, but it'll just have to do. If you like busy work, GNB's your guy. If I had to give a negative criticism about it, it's that it's probably the worst tank in dungeons, but even the worst can still get the job done just fine.

DRK's a solid tank. You'll never feel like you're lacking anything mechanically but is a bit on the boring side in my opinion because it plays slow like WAR without the payoff of seeing huge pretty numbers, and optimization can feel like a stressful endeavor because getting five hits under BW feels like a crapshoot even with the shorter GCD builds.