1. What does it mean?
겜 compresses 게임 into one syllable. It appears in quick messages about playing a game and in compounds such as 즐겜, playing for fun, or 겜알못, someone who does not know games well.
Also written or said as gem
2. The nuance
It is informal shorthand rather than a different type of game. Write 게임 in formal descriptions where clipped chat spelling would feel out of place.
3. Examples
“오늘 저녁에 무슨 겜 할래?”
What game do you want to play this evening?
“이 겜은 혼자 해도 재밌네.”
This game's fun even when I play alone.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.