Fascinating question. I would say that the Steel Defender will not be a spiritual creature, but a physical one, though one that only responds to your character. So the Gestalt for example can't command the Steel Defender to do anything without your character specifically making the same order, but it will recognize the Gestalt as an ally. Hence forth if your character is capable of summon/creating/having allies beyond themselves, such as a familiar, animal companion, or whatever, they'll largely be a physical being with all the risks that entails. So they could potentially be targeted by enemies, fall into traps, take damage, etc.