1. What does it mean?
고닉 shortens 고정닉. It is especially associated with forums that distinguish registered, persistent identities from more changeable or nonmember posting names.
Also written or said as 고정닉, gonik
2. The nuance
The exact account categories differ by site. A recognizable nickname does not prove a person's real identity or expertise, and the term can be used loosely for a familiar regular.
3. Examples
“이 고닉은 매주 전시 일정을 정리해 주네.”
This regular poster puts together the exhibition schedule every week.
“헷갈리지 않게 고닉 하나로 계속 활동하고 있어.”
I keep using one fixed nickname so people don't get confused.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.