Yikes! Gloom looks very creepy.
The mask has loads of detail in it, which really amps up the scare factor of this guy. A lot of that comes from the shading you've added in there to bring out the finer wrinkles and blemishes of his face.
To give the piece even more depth, you could try adding in some additional rendering that helps to describe the larger plane forms of his skull. Such as around the cheek bones, nose, and where the forehead transcends into the brow.
Keep up the great work.