We have no dates for Guilin, but the fact that he had sons who fought in the later Wars of Beleriand shows that he must have been extant before about the year I 400 (and he might quite conceivably have been alive during the Years of the Trees). He makes no direct appearance in the tales of the First Age, and his sons appear to have inherited his rank in Nargothrond (though this is ambiguous). We might take this to mean that Guilin was slain in the Wars against Morgoth, or perhaps even that he remained behind in Aman when his sons followed Finrod into Middle-earth.

Both of Guilin's sons, Gwindor and Gelmir, were accounted nobility within Nargothrond. This seems to imply that Guilin himself was a prince or lord of Nargothrond, or at least an important follower of Finrod in Aman, but this is never stated explicitly.

The origins of Guilin's name are difficult to discern. It seems most likely to be related to the word cuil, 'life', though it may alternatively have a connection to gui-, 'everlasting'. The fact that the name contains ilin (a Quenya word for 'pale blue') seems unlikely to be linguistically relevant, though it is interesting to note that both of Guilin's sons apparently also had words for 'blue' in their names.