Creating an animated flowing curtain style motion background effect

Create a gradient as shown.

Create a new file with 600X300. Apply gradient diagonally.

Now create a new file of 300X200 pixels.

Drag the gradient layer to the 300X200 pixel file.

Create a new layer. Apply gradient from left to right.

Change the layer mode to "multiply" for this layer.

Open animation window.

For the first frame select the dragged layer. Press Ctrl+T and move the layer as shown.

Duplicate frame. Press Ctrl+T. Move the layer to right.

Click tween form the bottom of the layer palette. Apply settings as shown.

 

Press Ctrl+Alt+Shift+S to save an optimized gif.