While they've really done a decent job with DRK's offensive rotation and a great job with GNB's offensive rotation, what is the theme with the defensive identities? PLD has blocking, covering, and healing. WAR has damage return, heavy burst, and mini-bloodbath. GNB and DRK have almost nothing, and there's where the discontent stems from with these two jobs.
They feel incomplete.
All of GNB's mitigation revolves around "Press this button for x amount of mitigation for x amount of seconds" which is absolutely no better than Bulwark or Sentinel. And all of DRK's mitigation skills are the same except for Blackest Night. They both need to undergo severe defensive and, if necessary, offensive reworks to complete the jobs.
GNB:
What's the lore/story behind why they are tanks? Well first off, they're bodyguards. And another thing they said is that they don't use guns like MCH or Garlean Gunblades for ranged damage. Instead they fire powder shells that have special properties to them, such as healing or shielding.
Suggestions
Give GNB an expanded take on the lost 'Reload' mechanics from MCH. Give GNB three types of ammunition types they need to keep reloading and expending: Healing Shells, Barrier Shells, and Explosive Shells. Take Blackest Night off of DRK and put it as Heart of Stone on GNB. It makes much more sense for GNB to be the barrier tank than DRK. Adjust mechanics accordingly. Allow for healing shells to be a mix of Cures or Rejen effects. Explosive shells stay put as their current offensive skills and rotation due to their offensive identity being pretty good tbh.
DRK:
The lore/story/theme behind DRK? Harder to pin down than GNB due to it being not just a Final Fantasy job, but a widely used concept in many stories and video games as a whole. But in Final Fantasy, DRK is known for being a Tank-ier DPS job that sacrifices its HP for damage rather than MP. It's also known for having Drain Blade and various other siphon skills and a few black magic spells. In non-FF stories, Dark Knights are often known as, for lack of better terms, assholes who simply do what needs to be done, have zero loyalty or bias for anything but the greater good, and often utilize techniques that let them come off at face value as total villains.
Suggestions
Let DRK be a mix of a Siphon-based tank and an auto-revive tank and throw in some double-edged(Give + Take) buffs to himself and party members. Remove Blackest Night as their main mitigation skill and put it on GNB where it more appropriately belongs. Let the majority of their mitigation come from Blood Weapon (All attacks return health). Change Darkside to convert MP costs into % based HP costs for extra damage. Change Dark Arts to increase MP costs ( thus Increasing % Hp costs w/ Darkside) for even more damage (which will inadvertently increase Bloodweapon's restorative effects to help mitigatate Darkside). Allow Living Dead to be put on a party member of choice for Auto-Revive, BALANCE ACCORDINGLY. And let skills like Sole Survivor be a buff to apply to a party member that kills their natural HP rejeneration and puts a rejen effect on the DRK, or maybe Dark Dance will put a Vuln-Up debuff on a party member while buffing your defenses/mitigation by a substantial amount to assist in tank busters.
These are just ideas so don't take them too seriously, but my point still stands. DRK and GNB have unthoughtful mitigation (Tank) identities, which is why there are so many more people concerned about them than WAR/PLD. WAR/PLD has their themes and they get their points across in a way that justifies them as tanks. Gunbreaker and Dark Knight do not. They really do just feel like DPS with high HP and a couple of flat mitigation skills.