To move the placement of a radial gradient in the object, drag the square handle. When the radius is the same for both circle handle, the gradient is perfectly circular. Three handles are available : the square one is the center of the gradient, and the circle ones change the radius of the gradient. Radial gradients work as colored circles with the color transition beginning at the center. The two handles can now be used to change the color (select each one and choose the color for it from the fill and stroke dialog) or move and change the gradient direction. Two handles now appear: the handle of the beginning is square and the end is a circle. Just click where the gradient should begin, hold and drag, and release where it should end. Linear gradients transition the color change from one point to another in a straight line. To invoke, either click the Gradient Tool icon or press Ctrl + F1. The gradient can be set in the Fill and in the Stroke Paint dialog to replace of any other type of coloration.

You can drag these handles directly in the drawing, to interactively adjust gradient positions. Any number of selected objects can simultaneously display handles and direction lines for the linear and radial gradients in their fills or strokes. It can be used on any shape, closed or opened, as a fill or a stroke (applied separately). The Gradient Tool allows the progressive passage of one color to another (or multiple others) in an object.

Inkscape gradient box