So I never noticed the Survivalist Pack until I saw something about it on the Wiki. I went up to Red Gate to find the pack and the rifle wasn't in it... I'm guessing a white legs guy picked it up and since I missed it I'm guessing its gone... Is there any hope of finding it?

If you're playing on PC you can spawn it? If you're on console, then hunt down every White Legs tribal in the area.

Thats the problem I left after I killed Salt Upon Wounds and came back and all the bodies are gone...

Then I'm afraid you're screwed, since whoever picked it up is gone for good. You might try reloading a save from before you drove out the White Legs and seeing if you can get it then.