1. What does it mean?
국룰 blends 국 from 국민 with English 'rule.' It presents a familiar habit or expected choice as though everyone naturally knows and follows it.
Also written or said as gungnul
2. The nuance
It is not an actual law or proof that all Koreans do something. Speakers often use it to promote a personal preference or an in-group custom with mock authority.
3. Examples
“우리 모임은 늦는 사람이 먼저 연락하는 게 국룰이야.”
In our group, the unwritten rule is that whoever's running late messages first.
“비 오는 날 이 노래 듣는 건 내 마음속 국룰이지.”
Listening to this song on rainy days is my own personal rule.
Sources
National Institute of Korean Language
References support the meaning and usage. Example sentences are written for NoCapWiki.