PresetLibraryHelper.cs

using System;
using System.Reflection;
using UnityEditor;
using UnityEditor.Callbacks;
using UnityEngine;

public class PresetLibraryHelper
{
		public static Type presetLibraryType = GetType ("UnityEditor.PresetLibrary");

		static Type GetType (string typeName)
		{
				return Types.GetType (typeName, "UnityEditor.dll");
		}

		public static string GetName (object presetLibraryObject, int index)
		{
				MethodInfo info = presetLibraryType.GetMethod ("GetName", new []{typeof(int)});
				object obj = info.Invoke (presetLibraryObject, new object[]{index});
				return obj != null ? obj.ToString () : "";
		}

		public static int Count (object presetLibraryObject)
		{
				MethodInfo info = presetLibraryType.GetMethod ("Count");
				object obj = info.Invoke (presetLibraryObject, new object[0]);
				return obj != null ? (int)obj : 0;
		}

		public static T GetPreset<T> (object presetLibraryObject, int index)
		{
				MethodInfo info = presetLibraryType.GetMethod ("GetPreset", new []{typeof(int)});
				System.Object obj = info.Invoke (presetLibraryObject, new object[]{index});
				return obj != null ? (T)obj : default(T);
		}
}

Test.cs

using UnityEditor;
using UnityEditor.Callbacks;
using UnityEngine;

public class Test
{
  		[DidReloadScripts]
		static void OnDidReloadScripts ()
		{			
				Color color = DoTest<Color> ("Assets/Editor/EditorColors.colors");
				Debug.Log (color);
				Gradient gradient = DoTest<Gradient> ("Assets/Editor/EditorGradients.gradients");
				Debug.Log (gradient.colorKeys);
		}

		static T DoTest<T> (string path)
		{
				Object presetLibraryObject = AssetDatabase.LoadAssetAtPath (path, PresetLibraryHelper.presetLibraryType);
				if (presetLibraryObject == null)
						return default(T);
				int count = PresetLibraryHelper.Count (presetLibraryObject);
				Debug.Log (count);
				string name = PresetLibraryHelper.GetName (presetLibraryObject, 0);
				Debug.Log (name);
				return PresetLibraryHelper.GetPreset<T> (presetLibraryObject, 0);
		}
}