Paladin misunderstanding.

[Sapphidia]

I tried with PLD, but I really find it to be by far the worst tank of the lot, from a tanking stand point, because GNB may be squishy, but at least it doesn't give you utility you aren't supposed to use, it has a plethora of CDs (though weak) and good damage.

I've mained Paladin since the start of the game, and play Gunbreaker as my secondary tank job for variety. I think there must be something going wrong here with the way you've setup your skills or use them because I really feel the only lacking thing from a Paladin's toolkit compared to GNB is Camouflage, and that's matched by your passive block abilities.

Paladin to me is miles ahead of Gunbreaker in general survivability when needed, both of itself and its party. Gunbreaker's advantage is primarily in damage.

Both classes have Rampart (20% damage reduction for 20 seconds).
Both classes have Sentinel (30% damage reduction for 20 seconds).
Both classes have Arms Length (20% attack speed reduction for anything hitting you for about 20 seconds).
Both classes have Reprisal (-10% damage done aoe debuff, for 10 seconds).
Both classes have Low Blow and Interject to stun/interrupt mob abilities.
Paladins have Hallowed Ground, Gunbreakers have the objectively worse Superbolide.
Paladins have a constant passive chance to block any attack. Gunbreakers have Camouflage on a cooldown.

Those are the obvious match-ups in mitigation. Now for the major differences.

Gunbreakers have Heart of Stone (15% damage reduction for 7 seconds once every 25 seconds). Paladins have Sheltron (15% damage reduction via blocks for 6 seconds every 50 Oath gauge). The oath gauge takes about 20 seconds to get to 50, but has the advantage of being able to stack to 100, so you can use two Sheltrons back-to-back if you get consecutive big hits. It's much more versatile once a fight starts, but just prevents very early use. Advantage Paladin due to versatility of this move.

I wont mention Cover or Intervene, because these cost 50 oath gauges and are the equivalent of "casting a Sheltron on a friend", and the Gunbreaker can just cast Heart of Stone on friendly targets, so these kind of balance out, though a Paladin can outright save someone with Cover + Hallowed Ground whereas a Gunbreaker just has the damage reduction option.

Gunbreakers get Aurora which is a free small heal over time every 60 seconds. It's good, as its free and you can keep it up constantly, but its only a fairly small amount of health every 60 seconds. Couple this with the small heal from Brutal Shell and when doing their basic single target rotations, Gunbreakers self heal a reasonable amount of light incoming damage without their dps rotation suffering. Paladins get Clemency, a skill they never use when things are going well. But when things go to hell-in-a-handbasket, Paladins can throw out Clemencies enough to get them up to full health many times over. Countless times in dungeons I've had healers die to mechanics and as a Paladin it's been cake to switch into Healer Mode and keep myself AND the other dps up. Gunbreaker just has to hope that its Shell+Aurora is strong enough to outheal things.

Gunbreaker has Heart of Light, which is a 10% magic reduction buff to the whole party. This is good. However, its use is to reduce AOE damage on big telegraphed attacks, and generally on these attacks its always the DPS or healers that are at risk of exploding. It's rarely the tank that dies to big AOEs in bosses because the AOEs are tuned to be survivable by a squishy DPS. The Paladin equivalent, DIvine Veil, gives 10% of the Paladins Health as a shield to everyone else in the party. Sure it doesnt help the Paladin but it's arguably slightly better at doing what this kind of move should do - keep the squishies alive.

That's everything both classes have access to defensively in a 1:1 comparison.... and the Paladin ALSO has Passage of Arms (15% damage reduction to themselves and everyone behind them) AND Shield Bash which allows them to tab->stun->tab->Stun huge packs of normal mobs for 6 second stuns each, which is a fantastic emergency method of mitigating damage if something bad happens.

When I compare the above, I just cannot see how someone can view the Gunbreaker as the toughest of the two tanks. They're incredibly similar, and whilst one may favor the "passive" healing from Brutal Shell and Aurora of the Gunbreaker, nothing comes close to the Paladin's ability to keep themselves and multiple dps up in an emergency via swapping to Clemency spam (though of course you hope never to need this as in the toughest of tough Ultimate/savage fights, if you have to do this then it means you wont meet the dps checks).

So the main variety is in their "healing options", and as almost every veteran will tell you, Healing is not something tanks should be doing if all is going to plan. Healing isnt mitigation. I only include Clemency and its awesomeness at survivability from the perspective of things like "healer disconnected on a dungeon boss" or "both healers died and you needed to self heal a tank buster whilst the RDM raised one". Those situations are basically guaranteed wipe on the hardest encounters anyway, but at the lesser end of things the Paladin comes across as borderline overpowered in survivability. And that does count for something as not every encounter is the top Savage bosses... but even discounting Clemency, hopefully the 1:1 comparisons above show that the Paladin has just as much self mitigation and significantly higher group utility, just at the cost of doing a bit less max dps than the Gunbreaker.