This tool is found about 3/4 way down the tool bar.
Make a selection, take the paint bucket tool and set opacity = 100 and tolerance = 255
Click anywhere on the picture and the selection will be filled with the foreground colour.
If you reduce the opacity then the selection gets filled with a semi-transparent version of the foreground colour so that the original image still shows through.
Reduce the tolerance and only those parts of the selection which are a similar colour to the spot where you clicked, will get filled.
It is probably easiest to make a selection first rather than relying on the colour matching.
Found just below the paint bucket icon in the toolbar.
Set the foreground colour to white. Take the gradient tool, set the main option to 'foreground to transparent' and click on the 'linear gradient' icon (of the 5 little flag like icons in the options bar).
Now click on your image and drag a line across it. You will see that your image (or your selection) has had a gradient painted on it - varying from white where your line started to transparent where it ended.
If you reduce the opacity this is quite a nice tool for producing a misty effect.
There are several other preset gradients though the only other one you are likely to use is the foreground to background. If you use dark blue as the foreground colour and light blue as the background this can be useful for improving a grey sky.
You can also set radial gradients and other shapes.