General overview

Gnosis' banner introduced the cold/freeze mechanic, which his kit uses to full effect. Freeze is an incredibly powerful status effect on its own because it stops enemies from acting and has a built-in RES debuff, but he introduces damage amplification to the mix to make it hurt even more than necessary. He's surprisingly good for general use and can serve a variety of unique roles depending on what skill you use. His only downside is that he can't debuff anything that's freeze-immune, but there are so many terrible enemies that aren't, and you'd be hard-pressed to find a more efficient (and probably unethical) way to deal with them.

Before I am accused of bias here, I want to clarify that I don't like Gnosis. This man broke into my house and robbed me.

E2? Gnosis certainly isn't an early-game fast track E2, but, like the other members of his archetype, he requires it at some point to be used effectively. His primary means of debuffing, fragile, is almost doubled at E2, and his skill 3 is busted. His module is also busted.

Talents

Talent 1 (Rigid Ice) is central to Gnosis' kit because it lets him inflict Cold when he attacks enemies. He also inflicts 25% Fragile on Cold enemies at E2, and enemies with Freeze receive double the effect - so 50% Fragile! This is currently the strongest Fragile debuff in the game. Also, Freeze comes with a -15 RES debuff! The tradeoff is that some bosses are immune to Freeze.

Talent 2 (Two Paths, One Goal) gives status resistance to all characters in the Karlan faction after he's deployed for 10 seconds. (This includes Gnosis himself.) Status resistance is never a bad thing.

Skills

S1 (High-Speed Cogitation) is an auto skill that has Gnosis' next attack hit twice with a damage bonus. He gets an instant Freeze out of it, and the fragile from his talent will apply for a little extra damage. It doesn't extend his Freeze duration, so it's more of a disruptor than dedicated crowd control.

S2 (Zero-Point Burst) deals Arts damage to all enemies in range and inflicts Cold for a few seconds. If you let it charge twice, it will inflict two stacks of Cold (i.e. Freeze) instead. It gives you an on-demand AOE freeze with a really short cycling time, so you can consistently interrupt enemy attacks, movement, or skills and give your team some breathing room. The tradeoff is that it has basically nonexistent damage, but that's not much of a tradeoff.

S3 (Hypothermia) is Gnosis' marquee skill. It more than doubles his ASPD, hits two enemies at once, keeps enemies frozen until the end of the skill (and prioritizes unfrozen enemies), then does one big hit of Arts damage at the end just because. The result is that he will make everything around him stop doing anything for a while. This is already great, but the skill description actually understates how powerful it is because you also get -15 RES and 50% Fragile from the Freeze. It can do everything except your laundry and probably your dishes.

Masteries

Prioritize S3 for sure, as it gets a huge amount of everything from masteries (ASPD, SP cost reduction, duration increase, damage bonus). Once you've done that, S2 is also worth looking at if you use it a lot because M3 decreases its SP cost and increases the duration on the Cold/Freeze. (It also increases the damage multiplier, but that's already pretty mediocre.)

Modules

Module X makes Gnosis even better at debuffing by giving him 10% Enfeeble on his targets for 2 seconds. The upgrades increase the Fragile effect from talent 1 to 28% at mod2 and 30% at mod3, which means he gets 60% Fragile for frozen enemies.