There are an astonishing number of things you can do with this tool, and the possibilities may seem a bit overwhelming at first. The two most important options you have are the Gradient and the Shape. Clicking the Gradient button in the tool options brings up a Gradient Select window, allowing you to choose from among a variety of gradients supplied with Gimp; you can also construct and save custom gradients. See the Help section on Gradients for more information.

If the gradient appears in black-and-white, then it will actually use the foreground and background colors instead (and you will see it colored that way in the tool options area). If it appears in color, then it will be used as is.

For Shape, there are 11 options: Linear, Bilinear, Radial, Square, Conical (symmetric), Conical (asymmetric), Shapeburst (angular), Shapeburst (spherical), Shapeburst (dimpled), Spiral (clockwise), and Spiral (counterclockwise); these are described in detail below. The Shapeburst options are the most interesting: they cause the gradient to follow the shape of the selection boundary, no matter how twisty it is. Unlike the other shapes, Shapeburst gradients are not affected by the length or direction of the line you draw: for them as well as every other type of gradient you are required to click inside the selection and move the mouse, but a Shapeburst appears the same no matter where you click or how you move.

[Tip] Tips
Check out the Difference option in the Mode menu, where doing the same thing (even with full opacity) will result in fantastic swirling patterns, changing and adding every time you drag the cursor.