The free image editor GIMP has a powerful Gradient Editor among its many features. This gives users the power to produce custom gradients. If you've ever looked at GIMP's Gradient Editor, you probably wouldn't describe it as very intuitive, and this may explain why many users make do with the preset gradients that come with GIMP. However, once you grasp the simple concept of how the Gradient Editor works, it is very easy to start building your own gradients.
The following few steps will show you how to produce a simple gradient that blends from red to green to blue. Once you understand the steps used to produce a gradient with three colors, you can use the same techniques to build more complex gradients with many more colors.