Does my Sett, the Boss's ability trigger when I use Deathgrip on my own buffed unit?

Yes, Sett, The Boss's ability can be used when you play Deathgrip on your own buffed unit.

Ruling

Sett, The Boss's ability is a replacement effect that modifies the "Kill" event. When you play Deathgrip, it contains a "Kill a friendly unit" instruction. If that unit is buffed, Sett's ability can intercede during the resolution of that instruction to replace the death with a recall.

Sequence of Events

1. Playing Deathgrip: You play Deathgrip targeting your buffed unit.
2. Resolution: As Deathgrip resolves, it attempts to execute its "Kill a friendly unit" instruction.
3. Replacement: Because the unit is buffed and would die, Sett, The Boss's replacement effect applies. You pay the cost (exhaust Sett and pay [C]), and the unit is recalled to your base instead of being killed.
4. Remaining Instructions: Because the replacement effect only changes the "death" event to a "recall" event, the rest of the Deathgrip spell continues to resolve.
   • Note: According to FAQ #8401, if you do not actually kill the unit (because the death was replaced), you do not give the Might bonus to the second unit. However, you still draw the card.

Important Nuances

• Replacement, Not Trigger: Sett's ability is a replacement effect, not a triggered ability. It does not use the chain and cannot be responded to once the cost is paid (FAQ #100, FAQ #5828).
• Buff Removal: As part of Sett's ability, the buff on the unit is spent (removed) (FAQ #5629).
• Deathknell: Because the unit was recalled and never actually sent to the trash, any "Deathknell" abilities on that unit will not trigger (Rule 734.1.d.1).