1. What does it mean?
ㄱㅊ commonly stands for 괜찮 or 괜찮아 in casual messages. It can reassure someone, accept a small inconvenience, or answer a question about whether something is all right.
Also written or said as gwaen-chan-a
2. The nuance
Initials are not unique definitions; other communities may use them differently. For an important decision, write the full response instead of relying on two consonants.
3. Examples
“조금 늦어도 ㄱㅊ. 나 아직 준비 중이야.”
It's okay if you're a little late. I'm still getting ready.
“이 자리 앉아도 ㄱㅊ? 친구 기다리는 거 아니지?”
Is it okay if I sit here? You're not waiting for a friend, right?
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.