I think Deck is an amazing rep! He has the implanted memories of human Deck so he also has the insecurities and self doubt of human Deck. He knows the boundaries of his "human" body so he stays within his boundaries. I like to think that he was activated right after Holden was taken out by Leon, and hasn't lived long enough to know he can do amazing things. Deck takes a beating from Zhora (kick murder squad) and then Leon (can lift 400 pound atomic charges all day), then after a sip of vodka and a three minute nap he's fit as a fiddle. That doesn't sound human to me. A super being that doesn't know it's capabilities and is operating under the parameters of a weak human makes the movie more interesting and more human to me. I'm just hoping that 2049 leaves this question unanswered, so I can keep thinking Deck is a rep without an expiration date.
As for my thoughts on Gaff. He's a human that has been assigned to watch over rep Deck. Gaff is always in the shadows behind Deck, but never seems to help-- even while searching Leon's apartment he does nothing to help Deck. He's only there as a handler. Why did Gaff let Deck live after his job was done? Same answer as why did Gaff let Rachel live-- who knows?
Fans have been divided over this issue for so many years, and there is good evidence for both sides. I think you should just believe whatever makes the movie better for you. When I saw the original release I had zero thoughts that Deck was anything but human. Then I saw the re-release where the unicorn was added, and it blew my mind to think that Deck could be a replicant!