VerticalGradient.cs

using UnityEngine;
using UnityEngine.UI;

public class VerticalGradient : BaseMeshEffect
{
    public Color topColor = Color.white;
    public Color bottomColor = Color.white;

    public override void ModifyMesh(VertexHelper vertexHelper)
    {
        if (this.enabled)
        {
            UIVertex vertex = default;

            float[] t = new float[4] { 0f, 1f, 1f, 0f };

            for (int i = 0; i < vertexHelper.currentVertCount; i++)
            {
                vertexHelper.PopulateUIVertex(ref vertex, i);

                vertex.color *= Color.Lerp(this.bottomColor, this.topColor, t[i]);

                vertexHelper.SetUIVertex(vertex, i);
            }
        }
    }
}