'スクリプト名' is missing the class attribute 'ExtensionOfNativeClass'!　とエラーが出る。

僕はunityでシーンを移動しても値が変わらず、セーブ出来る機能を作りたいのですが、unityのエラーに
'GManager' is missing the class attribute 'ExtensionOfNativeClass'!
というエラーが出ました。
どうしたらいいのでしょうか？
教えてください。

c#
1using System.Collections;
2using System.Collections.Generic;
3using UnityEngine;
4
5[System.Serializable]
6public class GManager
7{
8    private const string SAVEKEY = "PLAYER-SAVE-KEY";
9    static GManager instance = null;
10
11    public int hpNum;
12    public int mpNum;
13    public int hpNum2;
14    public int mpNum2;
15    [SerializeField]
16    public int coin;
17    [SerializeField]
18    public int sandohp;
19
20    //hosiiti
21    [SerializeField]
22    public Dictionary<string, int> dict = new Dictionary<string, int>()
23    {
24        {"kaze",0},
25        {"bomu",0},
26        {"speeddown",0},
27        {"mpkotei",0},
28        {"mpzouka",0},
29        {"mpkyushuu",0},
30        {"hpkaifuku",0},
31        {"hpzouka",0},
32    };
33    //hosiitiowari
34
35
36    public static GManager Getinstance()
37    {
38        if (instance == null)
39        {
40            instance = new GManager();
41        }
42        return instance;
43    }
44
45    public void Save()
46    {
47        PlayerPrefs.SetString(SAVEKEY, JsonUtility.ToJson(this));
48        PlayerPrefs.Save();
49    }
50    public void Lode()
51    {
52        string playerJson = PlayerPrefs.GetString(SAVEKEY, JsonUtility.ToJson(new GManager()));
53        instance = JsonUtility.FromJson<GManager>(playerJson);
54    }
55}