A gradient is a set of colors arranged in a linear order. The most basic use of gradients is by the Blend tool, sometimes known as the “gradient tool” or “gradient fill tool”: it works by filling the selection with colors from a gradient. You have many options to choose from for controlling the way the gradient colors are arranged within the selection. There are also other important ways to use gradients, including:

When you install GIMP, it comes presupplied with a large number of interesting gradients, and you can add new ones that you create or download from other sources. You can access the full set of available gradients using the Gradients dialog, a dockable dialog that you can either activate when you need it, or keep around as a tab in a dock. The “current gradient”, used in most gradient-related operations, is shown in the Brush/Pattern/Gradient area of the Toolbox. Clicking on the gradient symbol in the Toolbox is an alternative way of bringing up the Gradients dialog.

A few useful things to know about GIMP's gradients:

The gradients that are supplied with GIMP are stored in a system gradients folder. By default, gradients that you create are stored in a folder called gradients in your personal GIMP directory. Any gradient files (ending with the extension .ggr) found in one of these folders, will automatically be loaded when you start GIMP. You can add more directories to the gradient search path, if you want to, in the Gradients tab of the Data Folders pages of the Preferences dialog.

New in GIMP 2.2 is the ability to load gradient files in SVG format, used by many vector graphics programs. To make GIMP load an SVG gradient file, all you need to do is place it in the gradients folder of your personal GIMP directory, or any other folder in your gradient search path.