Isn't this almost the exact same injury that Perkins needed shoulder surgery for a few seasons ago?
Pretty much. A shoulder separation / dislocation means the shoulder popped out of joint. I think there's a technical difference regarding how much of the shoulder dislocated (i.e., a "partial" separation), but it's the same injury, just different to the matter of degree.
There's no guarantee that Gay will need surgery, though. Basically, once a guy gets a dislocation, it makes it likelier that another dislocation will occur. With each dislocation, the tendons / muscles holding the shoulder together weaken, holding it in place less and less tightly. Eventually, surgery may be needed to re-tighten the tendons. It's a pretty minor procedure, and can be done arthroscopicly now (as opposed to the more painful open surgery of the past).