Simply put, a gradient image is an image starting with one colour and gently (as gently as you want or as sharply as you want) merges through the image into another colour (or more colours)
The images below show just SOME examples of what you can do using a simple straight line using the Gradient Tool - where you begin and end the line will naturally produce different gradient images.
Also, the gradient tool comes with a number of different options which can be used to produce circles or diamond effects as you can see below...
These are just a few things to give you an idea of what can be achieved with simple gradient images....
Note, the single images below are the .GRD files which you can download individually by clicking on the filename SCn.GRD or you can download all 6 using the .Zip file at the bottom of the page...
To add the .GRD files into Photoshop, copy them into the ProgramFiles/Adobe/Presets/Gradients directory.
Go into Photoshop and click on the Gradient Tool. Then click on the box showing the current gradient and click on the small arrow inside the circle.
Then click on Load Gradients and you will see the list in the Gradients diretory and you can load the ones you want.
To download a .ZIP version of .GRD files above, click the link below: