I'm trying to create a custom blend mask in GIMP, but I'm not sure how to do it. I'm assuming you need to change the area that is transparent. 

Black is completely transparent, and will show the underlying skin texture of the character.
White is completely opaque, and will show your Headshot texture.
Gray will be a blend. So a 50 percent gray would be a 50 percent blend between the Headshot and underlying skin textures. A 20 percent gray would be 80 percent your Headshot texture and 20 percent the underlying skin texture.