In fact it is an art piece by Sol LeWitt at the Albrigh-Knox that gives us a 3D experience. Although knowing it is really, flat you assume it is a simple gradient, but in fact the gradient is made with a pattern of equally thick squiggled lines spaced to provide the 3D effect (detail below).
I have no idea how the seamless panels were imaged, or printed, or mounted. The description reads "Fiberboard with digital image on paper and styrene strip." Regardless of how it was done it creates a fascinating effect.