Each gradient fill contains settings that control the opacity of the fill at different locations on the gradient. For example, you can set the starting color to 100% opacity and have the fill gradually blend into an ending color with 50% opacity. The checkerboard pattern indicates the amount of transparency in the gradient preview.
Drag the corresponding opacity stop to the left or right.
Select the corresponding opacity stop, and enter a value for Location.
Drag the diamond above the gradient bar to the left or right.
Select the diamond and enter a value for Location.