The Basics

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The GIMP Blend tool enables you to make gradients. For instance, a gradient may be a paint fill or stroke which smoothly blends from red to green. A gradient may have sudden changes as well. And a gradient may be applied to selections or opacities.

On the right side of the gradient dialog box, are the default settings:

Mode - you can select mode here (more information in the Modes article)
Opacity - set whether gradient will be transparent or fully visible
Gradient - type of gradient
Offset - effects how far along the start colour in the gradient begins
Shape - shape of gradient
Repeat - way of repeating
Dithering - helps reduce banding by dithering the gradient.
Adaptive supersampling - try to refine the gradient in high contrast areas. Choose how much difference is required before supersampling activates, and how hard GIMP tries to get good quality samples.

Opacity

This bar controls the transparency of the gradient. Any value between 0.0 and 100.0 is possible with 100 being fully opaque and 0 being fully transparent. The default value is 100.

Gradient

There are hundreds of types of gradients in GIMP. Neons, flags, flares and much more. All of them have their own effect, but only few use selected colours. Here's short description of them:

FG to BG (HSV anti-clockwise) - using HSV palette, moves counter-clockwise
FG to BG (HSV clockwise hue) - using HSV palette, moves clockwise
FG to BG (RGB) - using RGB palette
FG to Transparent - moves from foreground colour to transparent.

Next to it you can see a flip option (). It reverses the gradient direction. For example, Red to Green would flip to Green to Red.

Offset

Gradient offset ranges from a default of 0, up to 100 percent. When offset is 0, the gradient begins at the start of the blending line. When offset is 50, the gradient begins at the halfway point in the blending line. Any earlier points are rendered with the same colour as the leftmost colour in the gradient. Setting offset to 100.0, which makes the gradient a solid (non-gradient) colour.

Shape

There are eleven gradient shapes, clicking and dragging inside a selection makes the gradient follow its boundary.

Linear - Smoothly blends from left to right
Bi-linear - Smoothly blends to the left and right from the middle
Radial - Linearly blends outward along radius - pseudo-spherical appearance
Square - Linearly blends into a square from center to corner
Conical (symmetrical) - blends linearly with angle in a circular arc
Conical (asymmetrical) - blends linearly with arc, until reaching the starting radius
Shaped (angular) - formerly shapeburst, produces an angular beveled gradient
Shaped (spherical) - fills selection with a bulging beveled gradient
Shaped (dimpled) - fills selection with a puckered beveled gradient
Spiral (clockwise) - clockwise spiral blend centered at click, with varying amount of twist
Spiral (counterclockwise) - counterclockwise spiral

Repeat

Example of triangular wave ( using radial shape ).

There are three options for repeating:

None - no repeat
Sawtooth wave - once passing the end of the gradient, begins again at the start
Triangular wave - once passing the end of the gradient, blends backwards until the start, and then begins blending forward again..

You can use Repeat with these shapes: Linear, Bi-linear, Radial, Square, Conical (sym), Conical (asym).

Dithering

Dithering reduces banding patterns in the gradient. If your gradient includes deliberately banded areas, turn dithering off in order to preserve them. If the result must be very precise, consider turning dithering off also (dithering trades spatial resolution for colour resolution, which is only good if you have spatial resolution to spare.)

Adaptive Supersampling

Adaptive supersampling identifies areas that seem to have high contrast and tries to improve the precision of the rendering only at these points. The general effect is that areas of high contrast are smoothed out; One easy way to see this is comparing the appearance of a 'sawtooth wave' repeated gradient with and without supersampling enabled. The options in this section allow you to control

How much contrast is considered 'high contrast'
How much GIMP will try to improve the sample quality before accepting the result (this comes into action when the additional samples' relationship to each other is also considered as high contrast)

See supersampling for a detailed explanation

GIMP/Gradient