Once you have a brush painted into a region you can use the Brush Transform tool (B) to visually alter the angle and other transform settings. Simply grab the head, tail or body of the brush transform arrow and move to see the gradient change. Rotating is easy too. Move the mouse near the head/tail of the arrow, when you see the rotation icon appear hold the mouse button down and spin the arrow.
Hold the Shift key down while spinning and the rotation angle is constrained to 15 degree increments.
[...] the editing of your tile or gradient brushes. I’ve written about this tool before (Constraining the Brush Transform Tool, Using a TileBrush in Expression Blend). Using this tool you will move the gradient to [...]