[Gradient Tool]

I have watched many tutorials on the gradient tool, but I can't find one that explains how you select the line you've drawn for a gradient once you have clicked away from it. The videos draw a line to create the gradient, change colors, etc.  I get all that.  But suppose you click on another layer.  Then you want to make changes in the gradient pixel layer.  How do you select the line that was there before, so you can make changes?

 

Thank you very much,

Patrice