This tool allows you to edit the gradient on canvas, but it only works for vector layers.
If you have a vector shape selected, and draw a line over the canvas, you will be able to see the nodes, and th stops in the gradient.
Move around the nodes to move the gradient itself.
Select the stops to change their colour in the tool options docker, or to move their position in the on canvas gradient.
You can select preset gradient in the tool docker to change the active shape's gradient to use those stops.