Does a gear with deathknell die when the unit is killed or does the deathknell effect activate from the death of the unit and the gear survives?

When a unit equipped with a piece of gear that has a Deathknell ability dies, the Deathknell effect triggers because the unit itself has gained that ability.

According to RiftJudge FAQ #8441:

• Deathknell abilities found in the "Effect Text" (bottom box) of a gear are appended to the unit while the gear is equipped.
• The unit gains the Deathknell ability, not the gear itself.
• When the unit dies, the unit's Deathknell triggers.
• If the gear itself is killed in isolation (e.g., by a card like Salvage), it is no longer attached to a unit, so the effect is no longer active and does not trigger.

In summary, the Deathknell effect activates because the unit (which has gained the ability) is dying. The gear does not need to be "killed" for the effect to trigger; rather, the gear's effect is active because it is attached to the unit at the moment the unit dies. If the unit dies, the gear is typically detached and recalled to its owner's base during the next cleanup (Rule 435.1).