It is possible using Animation
(Timeline in CS6!) and keyframes.
- Set your gradient overlay to its starting position.
- Open the
Animation
window (Window → Animation), and expand the layer you have the gradient overlay on. Click on "style" - Position the animation cursor to the first frame, then place a keyframe (yellow rhombus)
- Move the animation cursor to the last frame, place another keyframe
- Staying on the last frame, move the gradient overlay to its desired end position.
I hope this explains it well enough, if not, I can upload an example PSD file.
(PS: I realize this question was posted 4 months ago, but others might benefit from this answer)