Linda's answered your question, but to add to that...
You can also click along the bottom of the gradient to add more color squares and make a gradient with more than two colors. Once you do that, you can click on an added square to slide it horizontally, changing the proportions of the colors.
And... That vertical line you show with the circle at one end and the square at the other? You can click those and drag them anywhere. The line sets the direction of the gradient; and the points set the starting and ending boundaries. They can be within your object, making your gradient over a portion of the object, or they can be outside your object, which can sometimes handy.
whoa, neat, thanks everybody!!! (: