Set your secondary color to white, select Gradient, turn the :AllColorChannels: to :AlphaChannelOnly:, and make the gradient. Keep it at linear.
You should be drawing the gradient on the same layer as the line, and they should be on a different layer than the background. For maximum results, you should be doing each line on a seperate layer and then merging them all when you're done. I like to keep my lines into three or so layers judging by their size.