Images with gradient poorly managed by Unity

Hello,
I’m currently creating a 2D game on Unity and I have a problem with images with gradients or transparency. Here for example, the beginning of one of my levels before importing it in Unity (PNG-8) :
http://i.stack.imgur.com/kFH47.png

Here when the image is imported into Unity:

We can see that the gradation is very poorly managed by Unity. What are the ways to fix this problem ?

@Robin1002

set the texture import settings as follows

texture type : Editor GUI

max size : 2048 (or whatever is the max size of your texture)

format : true color