Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 253

Reduce the opacity for the active layer ( wine glass shadow ) to
approximately 70%.
The Eraser tool can be used together with a large, soft brush and a reduced
opacity (about 10%) to produce transparent surfaces in the wine glass and its
base. Experiment until you're satisfied. Then fill the Background gradient layer
with a two-color gradient blend of your choice. Save your image.
Figure 3.68
The finished image with the Layers dialog and the Gradient tool options. The paths are set to invisible in the Paths dialog.