Gradient background

As with most things in Unity, there are a variety of ways of creating gradients.

1) From a performance standpoint, the best way is probably a shader. Take a look at this package in the Asset Store:

https://www.assetstore.unity3d.com/#/content/9403

If you poke around you may be able to find free version of the two shaders you need.

2) If your gradients are one color...light to dark...you can use any shader that _Color property and a grayscale texture of the gradient.

3) For a horizontal or vertical gradient only, you can use vertex colors. Vertex colors require a shader that supports Vertex colors. Few of the shaders that come with Unity support vertex colors, but there are a number in the Unity Wiki. Or for an absolutely minimal shader you can use this one:

 Shader "Custom/Vertex Colored" {
     Properties {
     }
     SubShader {
         Pass {
             ColorMaterial AmbientAndDiffuse
         }
     } 
 }

Create Unity Quad (GameObject > CreateOther > Quad). Attach a material with a vertex shader to the quad and then attach this script:

 #pragma strict
 
 var startColor = Color.red;
 var endColor = Color.blue;
 
 function Start () {
     var mesh = GetComponent(MeshFilter).mesh;
     var colors = new Color[mesh.vertices.Length];
     colors[0] = startColor;
     colors[1] = endColor;
     colors[2] = startColor;
     colors[3] = endColor;
     mesh.colors = colors;
 }

4) You can create a procedural texture...that is generate the bitmap at runtime. Here is a link to get you started.

http://answers.unity3d.com/questions/589329/how-do-i-create-a-radial-gradient-texture-mask-usi.html