The easiest way to make a gradient:
Click the "Gradient" Tab (wait for your gradients to load, if they haven't yet...).
Click the "Create new" tab.
Click "Create from image". Choose a picture with colors that you like. Double-click the picture, and JWildfire will create a gradient from it, which you will see in the regular gradient location, all the way to the left, as the current gradient. But it has not yet been saved!
Now, above the main "Flame Editor" window, select the Grdnt button, the "Edit Gradient" button. Your new gradient will appear in the "Gradient Editor".
Now you can press the "Save" button in the Gradient Editor, and save the gradient. It will be a .map file. Turn off the Gradient Editor with the "Edit Gradient" button.
The new gradient should be available in the "Gradient Library", after you press the "Rescan" button.
There are other ways to make and save gradients, but this is the easiest, as it uses only JWildfire.