Gollum
Gollum is a fictional character from J. R. R. Tolkien's legendarium. He was introduced in the 1937 fantasy novel "The Hobbit", and became an important character in its sequel, "The Lord of the Rings". Gollum was a Stoor Hobbit of the River-folk, who lived near the Gladden Fields. Originally known as Sméagol, he was corrupted by the One Ring and later named Gollum after his habit of making "a horrible swallowing noise in his throat".
Gandalf
Over the next 17 years, Gandalf travelled extensively, searching for answers on the Ring. He found some answers in Isildur's scroll, in the archives of Minas Tirith. But he also wanted to interview Gollum, who had borne the Ring for many years. Gandalf searched long and widely for Gollum, and often had the assistance of Aragorn. Aragorn eventually succeeded, and Gandalf questioned Gollum, threatening him with fire when he proved unwilling to speak. Gandalf learned finally that Sauron had forced Gollum under torture in Barad-dûr to tell what he knew of the Ring. This reinforced Gandalf's growing suspicion that Bilbo's ring was the One Ring.