The 
Select Gradient window allows you to define a simple linear gradient (a smooth transition from one colour to another). The window appears when you edit a gradient or change from a colour to a gradient.
The Preview shows the gradient as currently selected.
The gradient transitions linearly from Colour 1 to Colour 2 in the direction set by the Direction dial.
Click OK to accept the changes and close the window. Click Cancel to discard the changes.