It's short for "그렇군" and more like slang meaning "Oh I see" or "(Yeah) that's right". 그렇군 ☞ 글쿤 그렇군요 ☞ 글쿤요 그렇구나 ☞글쿠나 \^o^/

글쿤 is the shorten form of "그렇군요" this expression is often used in the following situation when you want to utter your agreement to someone's comments. "Oh so it is"