Faces appear in many works of Gitxsan art. The face represents the human and you may see a face in a piece of work, it may represent the human Spirit that is within that animal. In chilkat artwork, the face represents the human Spirit and you may see two faces within that artwork. The face also represents the Watcher. You can see these faces on the fins of a Killerwhale for example.
Faces were drawn on masks to represent the reason for the mask. The mask could be a mourning mask and you may see tears on the mask, or the mask could represent transformation where a face would appear on the front and the mask would open to reveal a secondary Spirit.
Each culture has their own personal story. If you are wondering about a mask, or what a face means, ask a member of that nation.