A phrase which stands for "good night". It is commonly used in the Rainbow Six: Siege community after killing an enemy of the opposing team.
It is commonly used after one-tapping, or headshotting, the opponent with one bullet. The phrase comes from the figurative aspect of "putting the enemy to sleep".
The phrase is looked down upon and is considered toxic.
It is lazy, rude, and indifferent term, however. A good way to show people that you don't care enough about them to write them full words. A decent person who genuinely care about others would never use this abbreviation.
Friend A texting: I need to sleep now. Take care! :)
Friend B: Gn!
Friend A: Thanks.....
*Friend A Thinking* Oh the effort......