1. What does it mean?
ㄱㅅ takes the opening consonants of 감사. It is a quick acknowledgment of a favor, answer, or useful piece of information, sometimes repeated as ㄱㅅㄱㅅ.
Also written or said as ㄱㅅㄱㅅ, 감사, gamsa
2. The nuance
Although the full word appears in polite expressions, the abbreviation itself is casual. For a substantial favor or a formal relationship, a complete thank-you will usually communicate appreciation more clearly.
3. Examples
“빈자리 있다고 알려 줘서 ㄱㅅ. 지금 신청할게.”
Thanks for letting me know there's a spot. I'll sign up now.
“ㄱㅅㄱㅅ 덕분에 잃어버린 파일 찾았어.”
Thanks a lot—I found the missing file because of your help.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.