Im assuming its not as simple as finding a texture with a gradient?! Look in SharedMedia and other places for a gradient style texture.
If thats not what you mean and you want to control and change the colour of both sides of the gradient, the only think I can think of is to use KGPanels as a background (where you can set both sides gradient colour and alpha) then use WA on the top to cover the panel with a right-to-left progress bar with a transparent background.