The Fallout wiki states that the rifle is sometimes given as a reward for completing the quest Deliverer of Sorrows. Daniel should give it to you. If he does not and you have Walking Cloud as a companion or Follows Chalk he will sometimes give it to them. I hope this helps.
I thought the Survivalist's Rifle was not a reward but a weapon you must "discover" at Red Gate. I did Deliverer of Sorrows and The Vault Wikia actually says that it isn't a reward but, like I said, a discoverable item.
Here's the link so you can check it yourself. http://fallout.wikia.com/wiki/Survivalist%27s_Rifle
And guys, the same thing happened to me. When I first discovered Red Gate there was a severely weakened Giant Cazador nearby and no obvious corpses. I thought "Maybe one of those Dead Horse Stalkers took it and got killed somewhere". I searched for two real life hours solid. Imo, it is in circumstances like this where Rex's Search and Mark Perk would actually be useful. Anyway, I didn't find it and I killed a ton of Dead Horses as "revenge" for losing, what was for all intents and purposes, MY gun.
Patch the gun back please Obsidian, it's my gun and no matter how "good" it is (or not), it is one of about 3 guns that I'm currently missing from my "collection".