If I attach an equip in unit, and oponent react killing ruined rex using deathgrip, the deathknell is resolves before unit is attached or after?

Deathknell resolves first — the gear doesn't attach until after it.

Sequence: you activate Equip (targets locked at finalization, since Equip's choice is a Target, Rule 818.1.b.1), then opponent plays Deathgrip as a Reaction, which sits on top and resolves first, killing Ruined Rex. When Rex dies to that Kill Instruction, his Deathknell is placed on the chain as a pending item before he hits the trash (Rule 428.1.a.1.b) — meaning it goes on above your still-unresolved Equip ability. So LIFO order is Deathknell (4 damage) first, then the Equip ability.

Nothing ever attaches the gear during the Deathknell resolution — the gear is only linked to the unit when the Equip ability itself resolves. If the Deathknell's 4 damage kills the unit you were equipping (Rex's Deathknell picks its target when the trigger finalizes on the chain, Rule 352.4.b), the Equip ability mistargets on resolution and does nothing. If your unit survives the 4 damage, the Equip resolves right after and attaches normally.