system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Why did Sauron need Gollum's help to find the Ring?

Sauron and the Nazgul learn that the One Ring is in the Shire because Gollum is captured and interrogated regarding its whereabouts.

Why did Sauron need to get that information from Gollum? How did Sauron not already know that Bilbo had the Ring? Bilbo had worn it much more recently than Gollum had.
gpt
Sauron doesn't automatically know the location of the One Ring. It makes the wearer highly visible to him and the RingWraiths, but they *still have to be in line-of-sight* to stand out.

The entire time the Ring was lost in the Anduin, he didn't know where it was. While Smeagol / Gollum had it, he didn't know where it was. He didn't know it was being used. He knew it wasn't destroyed, because he still clung to an existence.

Gollum was captured, coincidentally.

 He was odd enough to warrant investigation, being unlike anything that his captors had seen before.

 Someone figured out he was a Hobbit whose life had been greatly extended by magic — and from there it was easy enough for them to work out which magic had extended his life. 

Then, they pressed him for information as to where it had passed to.

Shire. Baggins.