I am using GIMP 2.8 and, so far, upon browsing the internet, I have not yet found and effect that works for me. There was a post here on this site right here Creating a shiny black plastic effect, but the link in referrence on this post is broken, so this post does me no good. I attached the image that I'd like to apply the glossy effect here:
Put your layer in a group (Layer>New Layer Group; drag the layer onto the group in the layers list). This group will contain your base layer and all of the shading effects you want.
Make a new transparent layer in the group on top (Layer>New Layer with the previous layer selected). This is for the main highlight, so keep it selected.
Change "Mode" in the layers list from "Normal" to "Screen", "Overlay", or "Grain Merge" (they'll give slightly different effects).
Use the selection tools (Lasso, Square, Ellipse) (or the Path tool + Select>From Path) to define the shape of your main highlight. The shape of the highlight defines the shape of the surface.
Select the Gradient tool, set the gradient (in the tool options panel) to "FG to Transparent" and the shape to "Linear" (you could use spherical if you're making a rounded surface).
Drag from the brightest park of your highlight to the darkest part. Use Select>None to get rid of the dotted selection effect. At this point it looks like this, which is about the level of the button in the linked post:
- You can turn down the "Opacity" slider for the highlight layer to make the shine less strong. You can also add more effects the same way (make a new transparent layer inside the group, change the blend mode, make a selection, and fill it with a gradient). I'd suggest finding a reference of the style that you're going for and trying to copy the effects. For example, here's a more complicated version using a black version of the glyph you linked:
The only additional techniques used here are creating selections from the original image shape (Layer>Transparent>Alpha to Selection on the "Original Image" layer), and using the Shrink, Grow, Invert, Feather tools in the Select menu item to adjust that selection.
You're welcome to download the source at cl.ly/1M202i3O231j
Hope that helps!