1. What does it mean?
ㄱㄱ represents 고고, from English 'go go.' It can propose starting an activity, encourage someone to proceed, or agree that it is time to leave.
Also written or said as 고고, ㄱㄱㄱ, gogo
2. The nuance
You need the surrounding conversation to know what is being started. In a game it may mean start the next round; in a group chat it may mean head to the agreed place. It is not a formal instruction.
3. Examples
“준비 다 했으면 연습실로 ㄱㄱ.”
If everyone's ready, let's head to the practice room.
“한 판 더 할래? ㄱㄱ, 이번엔 지도 바꾸자.”
Want another round? Let's do it—change the map this time.
Sources
FluentU: Korean text slang guide
References support the meaning and usage. Example sentences are written for NoCapWiki.