transparent gradient

I would like a rectangular block that goes from some color (such as green) on one edge, and slowly transitions to fully transparent on the opposing side. I currently added a custom upload one I use, but it would be a great addition to the default icon set, and not require my map to become private. I turn the brightness down to 0, and use it as a shadow on staircases to indicate which way is down. could be used to indicate depth of a body of water if you turn the hue to blue, etc. Lots of uses.