OED says that this use of “gruesome” is literary and dialectical, so I am adding the “archaic” tag.  To “grue” is to shudder or feel such horror that you shudder – a northern, Scottish, dialectical verb.

• 03.031 and even gruesome,

“grue, v.1.” OED Online. Oxford University Press, June 2015. Web. 25 July 2015.

“gruesome, adj.” OED Online. Oxford University Press, June 2015. Web. 25 July 2015.