How to create a custom texture importer to fix GUI alpha gamma for Unity

CustomTextureImporter.cs

using System;
using System.Reflection;
using UnityEditor;
using UnityEditor.AssetImporters;
using UnityEngine;

namespace Hwei.AssetTools
{
    [CustomEditor(typeof(TextureImporter))]
    public class CustomTextureImporter : Editor
    {
        private Editor m_DefaultEditor;
        private TextureImporter m_Target;
        private bool m_AlphaGammaFix;

        private void OnEnable()
        {
            var type = Type.GetType("UnityEditor.TextureImporterInspector, UnityEditor");
            var editor = (AssetImporterEditor) CreateEditor(target, type);
            var setAssetImporterTargetEditor = type.GetMethod("InternalSetAssetImporterTargetEditor", BindingFlags.Instance | BindingFlags.NonPublic);
            setAssetImporterTargetEditor.Invoke(editor, new object[] {this});
            m_DefaultEditor = editor;
            m_Target = (TextureImporter) target;
            if (string.IsNullOrEmpty(m_Target.userData))
            {
                m_AlphaGammaFix = true;
            }
            else
            {
                var settings = JsonUtility.FromJson<GuiTextureSettings>(m_Target.userData);
                m_AlphaGammaFix = settings.alphaGammaFix;
            }
        }

        private void OnDisable()
        {
            DestroyImmediate(m_DefaultEditor);
        }

        public override void OnInspectorGUI()
        {
            if (m_Target.textureType == TextureImporterType.Sprite)
            {
                EditorGUILayout.Separator();
                var newValue = EditorGUILayout.Toggle("Correct Alpha gamma", m_AlphaGammaFix);
                if (newValue != m_AlphaGammaFix)
                {
                    m_AlphaGammaFix = newValue;
                    m_Target.userData = JsonUtility.ToJson(new GuiTextureSettings
                    {
                        alphaGammaFix = m_AlphaGammaFix,
                    });
                }
            }

            m_DefaultEditor.OnInspectorGUI();
        }
    }

    [Serializable]
    internal struct GuiTextureSettings
    {
        public bool alphaGammaFix;
    }

    internal class CorrectAlphaGamma : AssetPostprocessor
    {
        private void OnPostprocessSprites(Texture2D texture, Sprite[] sprites)
        {
            if (string.IsNullOrEmpty(assetImporter.userData))
                return;
            var settings = JsonUtility.FromJson<GuiTextureSettings>(assetImporter.userData);
            if (!settings.alphaGammaFix)
                return;
            Debug.Log("Modify Texture here, reference to https://docs.unity3d.com/ScriptReference/AssetPostprocessor.OnPostprocessTexture.html");
        }
    }
}