Scanlines are basically a watermark, although I've never used them myself.
Gloss is really easy. There are several ways to do it by hand. You can take a brush and set the opacity to 10-20% and resize it to something large and justclick to form a circular glossy area, or do it all in a single stroke for whatever shape you want. Or, form an area and then color it in at that opacity. If you're making a head or just to be safe in an avatar, make a new layer so all's well if you decide you don't want it.
Also, I find gloss to be more appealing than scanlines, which were originally meant to prevent you from stealing someone's work. :/