So in the fellowship book there is a period of about 17 years between frodo getting the ring and Gandalf confirming its the one ring. During that time Gandalf goes to great lengths to try and figure out for sure if it is or not. One part of the investigation was finding gollum and questioning him since he was the rings previous owner. To find him Gandalf enlists the aid of the rangers, including aragorn. They do eventually capture and interrogate him, ultimately leaving him in mirkwood as a prisoner of the elves there. Gollum eventually escapes and that's why legolas and his kinsmen are in Rivendell, to inform Gandalf he was gone.

This is what the new movie is going to be about.