This word for twilight, or less commonly dawnlight, is related to “glow” and to the second word “gloom” – “heat of the sun”, from which we get “gloom-stove”, a drying oven for gunpowder.  This “gloom” is completely separate from the “gloom” which has to do with staring and the origins of “glum“.  Possibly from Old English “glóm”, but not for sure.

• 08.131 to the gloaming
• 19.003 On hearth in the gloaming

“gloaming, n.” OED Online. Oxford University Press, June 2015. Web. 26 July 2015.

“gloom, n.2.” OED Online. Oxford University Press, June 2015. Web. 26 July 2015.