[Gurthang]

If you're asking the question of 'did the sword talk or not', you would have to say that Tolkien made an error in his writings. It says that it did, and I for one take that at face value that the sword actually spoke. I'm not saying that you should take everything completely literally, but in this case I do. I don't think that the tale is a rumor, or else it wouldn't be there at all.

And why shouldn't it talk? Eru could do anything he wanted, so he could let a sword speak. After all, Huan spoke, and no dog that I know has done that. There's even a story in the Bible where a donkey speaks to Balaam. (Numbers 22:30) So I say, why couldn't the sword have spoken.

Now as to why it did. It seems that Tokien used different resources to provide useful information. In this case, he used Gurthang to show why Turin deserved to die. It would have been kinda pathetic for Turin to say "Oh, I killed Beleg and Brandir, so I should kill myself." It sounds much better if someone else tells him so. There was no one around, so the sword conveyed this message to him. It's there to add to the story, not make it unbelieveable.