After you reveal the line and put a check in the box you can then click on the line in the spot where you want your fade to start and a ball will appear on the line. Move to where you want your fade to end, click on the line there, another ball will appear. Drag that ball down to fade out. Obviously, if you want to fade up you would drag your first ball down and leave your second ball at the default position.
I would include a screen shot for you but I don't have GB installed on this machine, sorry.
HTH