Do I need to pay the deflect cost if a unit with deflect has a gear equipped to it, I play detonate to kill the equipped gear?

No, you do not need to pay the Deflect cost.

According to RiftJudge FAQ #734:

"Ruling: No, you can target attached equipment normally. Equipment cards have their own properties, not those of the unit they are attached to (the Top Most Card)."

Reasoning:

  1. Targeting: When you play Detonate, you are targeting the gear (the equipment), not the unit it is attached to.
  2. Deflect Interaction: Deflect only applies to spells and abilities that choose the permanent that has the Deflect ability (the unit). Because the gear is a separate game object from the unit, targeting the gear does not trigger the unit's Deflect ability.
  3. Equipment Properties: As noted in the FAQ, equipment cards do not inherit the properties of the unit they are attached to. Even if the unit has Deflect, that protection does not extend to the gear attached to it.