You can transform a linear gradient into a radial one by using the top row buttons in the Fill and Stroke dialog, or you can replace the old gradient by creating a new gradient with the gradient tool, with the option for radial gradients selected.

When you move the diamond-shaped handle along the line with the mouse, the colors of the gradient get different weights. This will immediately show on the canvas. This will create a diamond-shaped handle. To create a gradient that has more than 2 colors, you can double-click with the Gradient tool on the blue line that spans between the handles.

in the Fill and Stroke dialog, select a color, which will then be applied to the object, at the location of the handle, or click on a color in the palette to assign it to the selected gradient stop.

select one handle of the gradient (circular or square) with the Gradient tool.

To change a gradient's position, just move the square or the circular handle with the mouse. By default, the tool creates linear gradients with two stops (i.e. The square symbolizes the beginning of the gradient, and the circle its end.

There will now appear two new handles on the object, one square, one circular, which are connected by a blue line. First, you need to select an object that you want to have a gradient, then click-and-drag over the object with the Gradient tool. The Gradient Tool can be activated in the tool bar. Inkscape allows you to comfortably create and modify gradients on-canvas, but it is useful to always have the Fill and Stroke dialog within reach, to be able to modify the colors easily.

Inkscape background color