From IFWiki

From the Scots verb grue meaning "to shudder," also the root word of gruesome.

A legendary creature in Infocom games that would inevitably eat a player-character who spent too long in the darkness. [ES]

The reason for this was to discourage the PC from wandering between dark rooms and getting to places he shouldn't (or forcing the programmer to code lots of extra responses when the player tries to do things in the dark) in much the same way that the threat of falling into a pit did in the original Adventure. [DC]