Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 252

From within the image, drag the cursor to the right until the path is tilted
to a 30-degree angle or thereabouts. Then click the Shear button in the Shear
dialog to accept your changes.
Next, the Scale tool is used to scale down the path vertically in a similar
procedure. In the following step, use the Perspective tool to narrow the cup
of the wine glass. Play with the options until you've attained a perspective
shadow.
Use the Move tool to position the path. Select the option Move: Path and
also the Move the active path option in the Tool Options dialog to move a path.
For the next step, click the button in the Paths dialog to create a selection
from the wine glass shadow path. Access the Select > Feather menu item to
define a border feathering of approximately 25 px. Create and activate a wine
glass shadow layer. Then use the Bucket Fill tool or choose Edit > Fill with FG
Color to fill the selection with black. The layers should be stacked as follows:
wine glass > wine glass shadow > Background gradient > Background .
Figure 3.67
The Shear tool and the tool options for transforming paths