You gain life equal to the damage dealt to you this turn. Simulacrum deals damage to target creature you control equal to the damage dealt to you this turn.
Simulacrum is the source of the damage. If an effect needs to know a characteristic of the damage’s source (Protection from Black, for instance), it will see the damage coming from Simulacrum.
You can’t use Simulacrum on Loss of Life, just damage.
This is not damage prevention or redirection. It actually is life gain and newly dealt damage.