Unity C# Attribute to enable HDR Gradient fields in the Inspector.

GradientHDRAttribute.cs

/**
 * GradientHDRAttribute created by Chance Millar at Two Tails Games on 2018/05/08
 * Use freely. That's a license, right?
 * 
 * Attach attribute like so:
 *
 *  [GradientHDR]
 *  public Gradient _HDRGradient;
 *
 * Make sure this is placed in a non-Editor folder inside Assets.
 */

using UnityEngine;
using System;
using System.Collections;

/**
 * Use this [GradientHDRAttribute] Attribute on Gradient fields to enable HDR Gradient UI.
 */
public class GradientHDRAttribute : PropertyAttribute {}

GradientHDREditorDrawer.cs

/**
 * GradientHDRAttribute created by Chance Millar at Two Tails Games on 2018/05/08
 * Use freely. That's a license, right?
 *
 * Be sure to place in an Editor folder inside Assets.
 */

using UnityEditor;
using UnityEngine;
using System;
using System.Collections;
using System.Reflection;

/**
 * Custom Property Drawer for GradientHDRAttribute
 */
[CustomPropertyDrawer( typeof(GradientHDRAttribute) )]
public class GradientHDREditorDrawer : PropertyDrawer
{
    // Cache the reflected method of EditorGUI.GradientField().
    private static MethodInfo sGradientFieldMethod = null;

    /// Cache the parms to save the GC from itself. Make sure the third param is always "true", this is your HDR enabler.
    private static object[] sInvokeParams = new object[3] { null, null, true };

    public override void OnGUI( Rect position, SerializedProperty property, GUIContent label )
    {
        // If the attribute is assigned to a non-Gradient type, then get outta here!
        if( property.propertyType != SerializedPropertyType.Gradient )
        {
            base.OnGUI( position, property, label );
            Debug.LogWarning( "GradientHDRAttribute assigned to non-GradientType" );
            return;
        }

        // Do standard UI stuff.
        EditorGUI.BeginProperty( position, label, property );

        // Draw the label so we know the variable name.
        position = EditorGUI.PrefixLabel( position, label );

        // Draw the HDR Gradient Field.
        GradientField( position, property );

        EditorGUI.EndProperty();  
    }

    /// Makeshift EditorGUI.GradientField()
    private static void GradientField( Rect position, SerializedProperty gradient )
    {
        // Check to make sure we have the EditorGUI.GradientField() method.
        ReflectMethod();

        // Update the params with the gradient info.
        sInvokeParams[0] = position; // Rect Param
        sInvokeParams[1] = gradient; // Serialized Property Param.
        // The third param is "bool hdr", which is already set to true in the initialization. Set that to false, and no more HDR field!

        // Call EditorGUI.GradientField() via reflection!
        // (This invoke can technically be cached, but because it's editor code, you only need to deselect the object with this field to get the ms back.)
        sGradientFieldMethod.Invoke( null, sInvokeParams );
    }

    /// Cache EditorGUI.GradientField() if it isn't already.
    private static void ReflectMethod()
    {
        if( sGradientFieldMethod == null )
        {
            // We know the function we need is under EditorGUI, so get that type.
            Type gradientEditorType = typeof( EditorGUI );

            // Now we know the field is called "GradientField", but it's an internal static type. So binding flags must be Static and NonPublic (as other GetMethod types default to public instance).
            // There's _lots_ of overloaded GradientFields in the source, but we're only interested in one particular method. (There's only 2 HDR-enabling methods). So match the params for the one we want!
            sGradientFieldMethod = gradientEditorType.GetMethod( "GradientField", BindingFlags.Static | BindingFlags.NonPublic, null, new Type[] { typeof(Rect), typeof(SerializedProperty), typeof( bool ) }, null );
        }
    }
}

[GradientUsage(true)]
public Gradient сolor;

This one works for me.