When Noxious Gearhulk enters the battlefield, you may destroy another target creature. If a creature is destroyed this way, you gain life equal to its toughness.
Use the toughness of the creature as it last existed on the battlefield to determine how much life to gain.
If the target creature has indestructible, it isn’t destroyed this way and you won’t gain life. If it is destroyed but put into a zone other than a graveyard, you’ll gain life.