radiantanor:

Remember Gorlim, and in particular how he is remembered as “the Unhappy”?

Beren was the only survivor of his dad’s heroic last stand group, and the only one to see the “wraith of Gorlim” and receive his warning. Which means he must have been the one who relayed that part of the story to the Elves or other Edain, who then wrote songs and histories about it. Given the detailed description of Gorlim’s backstory, his reasons for the betrayal and the emphasis on the mitigating circumstances, the most likely in-universe explanation is that Beren made sure he would be viewed sympathetically.

“The Unhappy” is not a positive epithet, of course, but it’s a lot more flattering than some of the other possibilities. Gorlim is a far more sympathetic
character than other notable traitors like Maeglin or Grima, and I know it’s because he actually was a better person. But in-universe, the main reason anyone would know all that has to be Beren’s decision to tell his story this way, even though he lost the last of his family and companions because of that betrayal. Many people would not have been as kind to Gorlim’s memory in his place, even considering his obvious regret and the attempted post-mortem warning.