can ledros kill gear as an additional cost

No. Commander Ledros's additional cost requires killing friendly units, and a Gear is not a Unit — "Unit" and "Gear" are distinct card types (Rules 141, 148).

The only exception is a card that is both types at once (e.g., a Gear that's also a Unit) — multi-type objects qualify as "units" for effects like this (Rule 178.3). A plain Equipment like Seal of Rage or a non-Unit Gear can't be paid as Ledros's cost, even though gear is generally Killable.