1. What does it mean?
겜알못 compresses 게임 알지도 못하는 사람. It can describe unfamiliarity with a particular game's rules, tactics, or culture, or with games in general. Speakers often use it to preface a beginner's question.
Also written or said as gemalmot
2. The nuance
Calling yourself 겜알못 can be lighthearted. Calling another person that can dismiss their opinion, especially when they are simply learning or disagreeing with you.
3. Examples
“겜알못이라 이 아이콘이 뭔지 모르겠어.”
I'm clueless about this game, so I don't know what this icon means.
“겜알못 친구한테 규칙부터 설명해 줬어.”
I explained the rules first to my friend who doesn't know much about games.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.