there should be some low rocks you can jump up, it's hard to explain where exactly but it's about the only spot around there you can get up onto something, if you've unlocked the redgate marker then you should already be up where you need to be btw.
You're lucky. I went through the trouble to get there after beating the DLC, only to find out some White Leg had made off with the rifle. Now I have no idea where it is, even if it still exists. Fortunately, I'm playing a Unarmed/Melee character so it doesn't really matter.