Or is that supposed to cinch it?
He appears to take the revelation rather calmly, doesn't he?
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Opinions are divided, even among the people with immediate creative input:
Possible "human" explanations are that Deckard talked about the dream off-screen and Gaff somehow learned about it, or that it's a coincidence and Gaff intended it to symbolize Rachel.
If Deckard is a replicant, he may long have suspected it himself and is therefore not shocked.