how do i add gloss to, let's say, just over the nose? I can select the faces(geometry) and assign a different material, but, of course, that's not gonna look very good. I don't want the gloss to cut off sharply at the edges (of the faces.) So, how do i go about doing this? Do i have to use an image with an alpha channel?

Thanks!
I'm only fairly new to Blender, but I believe that if you paint the area you want glossy in Texture Paint mode with a new black image as the base (and the glossy part painted white) you can then save that image. Then create a Gloss Node with your newly created image as the Fac input and use a Mix Node to mix it with your skin node/s.