Step 1: Select the object/person/... that you want to give a shadow.
It all starts with a selection. The tool you use for the selection depend from photo to photo. I'll explain how I've made the selection of the example image. If you want to use an other selection tool you can skip until Step 2.
Once you've opened the image in Gimp activate the Fuzzy Select Tool.
The Threshold is at 15 (=standard value).
Click in the white area outside the object. In case you wonder why I'm making a selection of the background? In this image it is easer to select the background first and than invert the selection.
Hold down the Shift key to activate the "Add to the current selection" function and click all the white areas inside of the corkscrew.
Invert the selection by going to Select > Invert.
Step 2: Creating the layers needed for the shadow.
Go to Edit > Copy (Crtl+C).
Go to Select > Past into. After you've done this you'll see a Floating Selection layer in the Layers window.
Now add a new layer (Layer > New Layer). The Floating selection layer is now transformed into a normal layer. Because we've used this way of working the corkscrew in the new layer stands in the exact position as the original.
Now you have to duplicate this layer (Shift+Crtl+D). You need those two layers because one of them will be used as the shadow and the top layer will make it look as the shadow is behind the corkscrew as it should be.
Press Crtl+Shift+A to get rid of the selection.
Step 3: The beginning of the shadow.
Activate the middle layer.
Check the Lock Alfa Channel box in the layer window.
And fill it with black. If you don't see anything changing it is ok. Hide the layer above and you'll see that you have created the beginning of the shadow, don't forget to make the layer visible again.
Uncheck the Lock Alfa Channel box in the layer window.
Step 4: Transform the shadow.
Will the middle layer is active go to Tools > Transform Tools > Perspective.
Drag the left and right upper corner down as shown here.
Step 5: Give the shadow some blur.
Before adding the blur go to Layer > Layer to Image Size. I've you don't do this the shadow will get a sharp edge at the places where the shadow is close to the layer edge.
Go to Filters > Blur > Gaussian Blur. Change the horizontal and vertical blur radius to 20px.
Step 6: Give the shadow some gradient.
Go to Layer > Mask > Add Layer Mask.
Set black as the foreground color and white as the background color.
Activate the Blend Tool and use the FG to BG gradient.
Draw the gradient (while the layer mask is activate) like shown here.
And the shadow is ready.