Probably a few ways to achieve this effect but here's one method. Create a solid, apply Generate > Ramp Effect and create your gradient. Now use the text tool to create your letter and place it above your solid. Change the track matte of the solid/Ramp layer to "Alpha Matte"(press F4 if the track mattes aren't currently visible). Now animate the position of the solid layer, or animate the position of the start end of the gradient. Then parent the solid to the Text/Letter, so if you move it, the gradient will move with it.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.