This post will explain how to do fading (fade out a layer) in Gimp.
Assuming you have an image open...
- Add a new layer
- Draw the part you want faded
- Right click on the layer in the Layers panel/window and select 'Add Layer Mask', then click on Add on the Add Layer Mask dialog (making sure the default value of 'White (full opacity)' is selected).
- Select the Blend/Gradient tool and set the Gradient to 'FG to GB(RGB)' .
- On the layer click and drag a line where you want the gradient (fade) to start and finish. The first click is where it will be faded out and the second click is where it will start to fade.
- Right click on the layer in the Layers panel/window and select 'Apply Layer Mask'.