GNB Healing is also a steady 1200 every 30s from Aurora and 900 every 20s from HoC. Pit them together in the same 2 minute period and the healing is 10,800 (rounding up one cast of HoC) as well as a 20% increase in max HP (That can be harder to calculate its actual impact because when it falls off you can lose the health if you were full). That should be around 150,000 from my previous assessment of what healing does with my gear. GNB is third in healing for tanks and in a 2 minute window the disparity is still almost ten times the difference. Even if we assume 200 potency per target on a pull from a pull of 10 mobs that's 2,000 potency every minute adding only 4,000 making it a 2 minute window of 5200 healing, less than half of GNB still. The math is there, DRK just can't compare to the other tanks.
EDIT: Math error, wrong cooldown.
This being correct humbles what I was saying as a difference, but still eclipses DRK entirely, outside of AD usage of course. I wonder what the math is on damage reduction...