A simple helper class, which allows you to create radial gradients. This can easily be applied to a texture or to a noise function.

Gradient.cs

using UnityEngine;

public class Gradient {

    /// <summary>
    /// Evaluated a radial gradient at the given x,y position
    /// </summary>
    /// <param name="x"></param>
    /// <param name="y"></param>
    /// <param name="sizeX"></param>
    /// <param name="sizeY"></param>
    /// <param name="minRadius"></param>
    /// <param name="strength"></param>
    /// <returns></returns>
    public static float RadialGradient(int x, int y, int sizeX, int sizeY, float minRadius, float strength) {
        float gradientSize = (sizeX + sizeY) / 2;
        float distFromCenter = Vector2.Distance(
            new Vector2(sizeX * 0.5f, sizeY * 0.5f),
            new Vector2(x, y)
        ) - minRadius;

        if (distFromCenter < 0) {
            distFromCenter = 0;
        }

        return 1 - (distFromCenter / gradientSize) * strength;
    }
}