Did Gandalf know about Gollum before the Hobbit?

He found out that Gollum had the one ring in the past, since Bilbo found a magic ring in Gollum’s cave. Gandalf already had many suspecions about this ‘magic ring’ that Bilbo found since the One Ring was lost for millennia. He wanted to find Gollum to stop him from being captured by Sauron.

What does Gandalf say about the fate of Gollum?

I wish none of this had happened. ‘ Gandalf: ‘So do all who live to see such times, but that is not for them to decide. All we have to decide is what to do with the time that is given to us.

How did Gandalf know about Gollum and the One Ring?

As the other answers have said, Gandalf knew about Gollum from Bilbo. He suspected that Gollum’s ring was powerful. By catching Gollum and interrogating him, Gandalf began to suspect that the Bilbo’s ring might be the One and discovered where Gollum had found it. He went to Minas Tirith because the last he knew of the One was that Isildur took it.

Did Gollum simply walk into Mordor?

So yes, Gollum did simply walk into Mordor. His research in Minas Tirith was rather the other way round. As the other answers have said, Gandalf knew about Gollum from Bilbo. He suspected that Gollum’s ring was powerful.

What is the history of Gollum based on?

To muddle things up even further, much of Gandalf’s information is based on conversations that he had with Gollum himself. This means the bulk of Gollum’s history is dependent on facts that he, himself, reported and then Gandalf assembled into something like a recognizable narrative.

Why did Gandalf go to Minas Tirith?

By catching Gollum and interrogating him, Gandalf began to suspect that the Bilbo’s ring might be the One and discovered where Gollum had found it. He went to Minas Tirith because the last he knew of the One was that Isildur took it. He was trying to tie Gollum’s finding to the last known location of the One Ring and succeeded.