We don't require that a lens focus light, only that it bend light, as you said. From optics, a concave lens disperses light, sending it out at wider angles than it comes in. From astronomy, other examples of lensing include cases where a single image is broken up into multiple images, or is smeared out into a distorted version of the original. A common lensing effect is our ability to see more than 50% of the surface of a spherical body because light from just past the horizon is bent in our direction.
As far as the shape of the gas cloud plays in, the total mass and the center of gravity of the cloud are the primary concerns in determining lensing. As long as the cloud is sufficiently compact, the shape only slightly changes the otherwise spherical distortion of the lensed images.
Edit: regarding the difference between a glass marble and a glass cube: keep in mind that those objects change the path of light through refraction, while galaxy clusters change the path of light by curving space. Refraction depends on the refractive indices of two materials and an interface between them; gravitational lensing can occur in vacuum near a point mass. The two processes are very different, and we only call the latter "lensing" because the effects are like using a lens, not because the lensing body forms an optical lens.