1. What does it mean?
걍 compresses 그냥 into one syllable. It can mean doing something without a special reason, leaving something as it is, or choosing a simple option.
Also written or said as gyang
2. The nuance
It does not always express annoyance. Tone comes from the whole sentence: 걍 좋아 can be affectionate, while 걍 됐어 can sound dismissive.
3. Examples
“오늘은 걍 집 근처에서 밥 먹자.”
Let's just eat near home today.
“이유는 모르겠는데 이 노래가 걍 좋아.”
I don't know why; I just like this song.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.