In Blade Runner, I never understood why Gaff spares Rachael's life. The only thing he says about Rachael is:
It's too bad [Rachael] won't live... but then again who does?
This doesn't really help me answer the question. Gaff, presumably a blade runner, went to Deckard's apartment probably to retire the last replicant. Remember, people see replicants as only machines, not regarded like humans.
Replicants are like any other machines. They are either a benefit or a hazard.
Now given this, Gaff sparing Rachael's life is highly unlikely. Even though he knows Rachael is fellow comrade, Deckard's love interest, it's shown in this universe that sympathy doesn't really operate even amongst blade runners. Especially Gaff and Deckard - they don't seem to like each other at all.
So why does Gaff defy orders to retire Rachael and ultimately spare her life?
P.S. If your answer involves Deckard being a replicant, then please explain it in terms of both possibilities, i.e., Deckard = replicant and Deckard = human.