Cleanup the Results
Crop the image if necessary. Select the "Crop" tool and highlight the area of the image you wish to retain.
Next convert the transparent area of the image to white. Create a new layer and select the initialization to be "White." A new layer is created on top, making your image appear to be white.
Now, move the white layer down using the arrow button on the "Layers" palette.
With GIMP, how to fade is no longer a mystery to you. Before you stop, however, there are a couple more steps to complete.
First, you need to apply the layer mask. you do this by right clicking the layer mask on th "Layers" palette and then choosing "Apply Layer Mask."
With that done, you have now permanently applied the changes you made using your layer mask. Now, merge the remaining layers together. To do this, right click on the "Layers, Channels, Paths" palette and choose "Merge Visible Layers." In the "Merge Layers" dialog, choose "Expand as Necessary" and then click the "Merge" button.
That seems like it took forever, didn't it? Now that you have learned how to create a fade effect in GIMP, you will be surprised at how many times you will find this technique useful. Also, with just a little more practice under your belt, you will find that you will be able to create these fade effects faster than you did this time through. Take a final look at the image in GIMP. How to fade is now something you know.