Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 214

The Blend Tool Options

The Blend tool includes an option called Opacity that allows you to create a
fully opaque, slightly transparent, or fully transparent gradient.
The Mode drop-down list provides a selection of paint application modes.
Leave the default, Normal , which is opaque without mixing attributes.
Gradient allows you to choose from a drop-down list of gradient patterns.
You can also select the Reverse check box to interchange the foreground and
background colors.
The Offset value determines the “slope” of the gradient. When you
increase the offset value, the center of the gradient moves to one side (which
side it slopes to will depend on the direction chosen for the gradient).
The Shape drop-down list offers a choice of several gradient shapes.
Linear refers to a parallel color gradient. You'll be using a linear gradient in the
following exercise.
Here is a list of other shapes:
• Bi-linear : Proceeds in both directions parallel from the starting point;
mirrored color gradient.
• Radial : Gives the appearance of a sphere.
• Square : Renders an equal-sided pyramid.
• Conical (symmetric) : Renders a cone; the gradient is mirrored.
• Conical (asymmetric) : Renders a cone with a simple gradient.
• Shaped (angular, spherical, dimpled) : These shapes create color
gradients that adapt to the contour of a selection in the image.
• Spiral (clockwise or counterclockwise) : Renders spiraling gradients.

Figure 3.40
The Shape menu for gradient fills

Use the Repeat option to decide whether the gradient should be applied
once ( None ) or whether it should be repeated with hard transitions ( Sawtooth
Wave ) or soft transitions ( Triangular Wave ).
Dithering applies the dithering method on gradients. Dithering creates a
smooth color transition by dispersing the colored pixels.
Adaptive supersampling conducts an anti-aliasing to sharp color
transitions to avoid a stepped color rendering with harsh transition lines.
Set the gradient options for the sample image as follows:
Opacity = 100%
Mode = Normal
Offset = 0.0
Shape = Linear
Repeat = None