Add this script to any game object with an image component you're using as a fill bar colour the bar based on the current fill amount and a gradient set by you. Works in the editor for testing as well.

FillBarGradientScript.cs

using UnityEngine;
using UnityEngine.UI;

[RequireComponent(typeof(Image))]
[ExecuteInEditMode]
public class FillBarGradientScript : MonoBehaviour
{
    public Gradient gradient;
    Image fillImage;

    void Start()
    {
        fillImage = this.GetComponent<Image>();
        fillImage.fillAmount = 1;
    }

    void Update()
    {
        if (fillImage == null)
            fillImage = this.GetComponent<Image>();
        else
            fillImage.color = gradient.Evaluate(fillImage.fillAmount);
    }
}