face, expression
顏 centers on the face as the visible front of a person, especially the look or expression it shows.
顏 combines 彥 (a person with distinctive features) and 頁 (head), together suggesting the face or facial expression. The modern simplified form is 顔.
The left side 彥 suggests a person's distinctive look, and the right side 頁 is a head. Together they form the face, where expression appears.
For ガン, imagine a face with a stern expression like a gang leader: gang -> ガン, and that face shows no emotion.