solid to transparent gradient

Add a Material to your plane, and enable Material Transparency, set the Alpha to 0
http://i.imgur.com/o5NTi.jpg

For this material, add a new Texture, set its type to “Blend”
Enable the “Color” Ramp, set the color like you need
Now at “Inflence” enable Alpha

It should work
http://i.imgur.com/3QVw2.jpg