Dither gradients provided, but custom ones can also be made. The requirements for a custom dither gradient is that each "block" has the same width as the height of the image. So a 4x4 sized dither (just like the default psx_dither.png), the height of the image is 4 pixels, and the number of steps of dither is calculated automatically.
Make sure to disable compression on the dither textures, set them to Point filter, disable mipmaps and disable Non-power of 2 resizing.