The visible texture of the canvas is created by the lights that are cast over it and the paint, and this is what also creates the slight gradient over the canvas area. As a result it's not possible at this point to remove the gradient without removing the texturing. If you turn off the canvas lights from the Canvas Panel the gradient will vanish but you will also lose the texture of the canvas and the texture of the paint. You could potentially import a canvas image in to a layer behind your paint to maintain the illusion of texture, but there's no way to keep the real thing without the gradient at this time.
Matt
ArtRage UI
Ambient Design.