adobe illustrator - How do I apply a color gradient to a stroke?

I can see the color gradient when my mouse is over the stroke but once I move my mouse away, the gradient goes away with it. How do I apply the gradient, I have been at this for 3 hours?! I am using Illustrator CC.

My specific problem is that I cannot apply a color gradient to a stroke. It is a straight vertical line I am using as a time line.

This far, I have edited the colors I want the gradient to be and along the stroke line I can see the colored gradient I made, it has arrows along it for me to edit the colors further if I prefer and that's where I am stuck. I do not need to edit the colors and more, they are what I like them to be.

Now I am trying to move on to the next step but I do not know how to make the colored gradient stay after I am done editing it.