Error while saving game: SerializationException: Type 'UnityEngine.GameObject' in Assembly 'UnityEngine.CoreModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.

I've made a generic save system that you can save datas with keys and then load them back again. It was working fine but when I tried to save two lists it gave me the following errors:

 SerializationException: End of Stream encountered before parsing was completed.
 ...
 SaveSystemF.Load[T] (System.String key) (at Assets/Scripts/SaveSystem/SaveSystemF.cs:33)
 CollectableDiseases.LoadHastaliklar () (at Assets/Scripts/Wanderers/CollectableDiseases.cs:45)
 CollectableDiseases.Start () (at Assets/Scripts/Wanderers/CollectableDiseases.cs:19)

and:

 SerializationException: Type 'UnityEngine.GameObject' in Assembly 'UnityEngine.CoreModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.
 
 SaveSystemF.Save[T] (T saveObject, System.String key) (at Assets/Scripts/SaveSystem/SaveSystemF.cs:19)
 randomBuy.Save () (at Assets/Scripts/randomBuy.cs:93)
 GameEvents.onSaveInitiated () (at Assets/Scripts/SaveSystem/GameEvents.cs:23)
 randomBuy.buyTheThing () (at Assets/Scripts/randomBuy.cs:79)
 UnityEngine.Events.InvokableCall.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent/UnityEvent.cs:166)
 UnityEngine.Events.UnityEvent.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent/UnityEvent/UnityEvent_0.cs:58)
 UnityEngine.UI.Button.Press () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:66)
 UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:108)
 UnityEngine.EventSystems.ExecuteEvents.Execute (UnityEngine.EventSystems.IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:50)
 UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:261)
 UnityEngine.EventSystems.EventSystem:Update()

Here is the script where I try to save the list:

 public class randomBuy : MonoBehaviour
 {
 
     public int paraOyuncu;
     public int paraFiyat;
     int gelenPara;
 
     [SerializeField]
     public List<GameObject> hastaliklar = new List<GameObject>();
     public List<GameObject> yakalanmisHastaliklar = new List<GameObject>();
 
 
     CollectableDiseases hastalikci = null;
 
     void Start()
     {
         hastalikci = GameObject.Find("CD").GetComponent<CollectableDiseases>();
 
         hastaliklar = hastalikci.hastaliklar;
         yakalanmisHastaliklar = hastalikci.yakalanmisHastaliklar;
 
         GameEvents.SaveInitiated += Save;
 
         LoadMoney();
         LoadGelenPara();
         LoadBox();
 
         paraOyuncu += gelenPara;
     }
 
     public void buyTheThing()
     {
         if(paraOyuncu >= paraFiyat && hastalikci.hastaliklar.Count != 0)
         {
             //ürünü al
 
             paraOyuncu -= paraFiyat;
             paraFiyat += zamMiktarı;
 
             GameEvents.onSaveInitiated();
         }
     }
     void Save()
     {
         SaveSystemF.Save<int>(paraOyuncu, "Player Money");
         SaveSystemF.Save<int>(paraFiyat, "Box Price");
         SaveSystemF.Save<int>(hile, "Player New Money");
         SaveSystemF.Save<List<GameObject>>(hastaliklar, "Hastaliklar");
         SaveSystemF.Save<List<GameObject>>(yakalanmisHastaliklar, "Yakalanmis Hastaliklar");
     }
 
     void LoadMoney()
     {
         if(SaveSystemF.SaveExists("Player Money"))
         {
             paraOyuncu = SaveSystemF.Load<int>("Player Money");
         }
     }
 
     void LoadBox()
     {
         if(SaveSystemF.SaveExists("Box Price"))
         {
             paraFiyat = SaveSystemF.Load<int>("Box Price");
         }
     }
 
     void LoadGelenPara()
     {
         if (SaveSystemF.SaveExists("Player New Money"))
         {
             gelenPara = SaveSystemF.Load<int>("Player New Money");
         }
     }
 }

and this is the script where I try to load:

 public class CollectableDiseases : MonoBehaviour
 {
     [SerializeField]
     public List<GameObject> hastaliklar = new List<GameObject>();
     public List<GameObject> yakalanmisHastaliklar = new List<GameObject>();
 
     void Start()
     {
         LoadHastaliklar();
         LoadYakalanmisHastaliklar();
     }
 
     void LoadHastaliklar()
     {
         if (SaveSystemF.SaveExists("Hastaliklar"))
         {
             hastaliklar = SaveSystemF.Load<List<GameObject>>("Hastaliklar");
         }
     }
 
 
     void LoadYakalanmisHastaliklar()
     {
         if (SaveSystemF.SaveExists("Yakalanmis Hastaliklar"))
         {
             yakalanmisHastaliklar = SaveSystemF.Load<List<GameObject>>("Yakalanmis Hastaliklar");
         }
     }
 }

also my saving script:

 public class SaveSystemF : MonoBehaviour
 {
     public static void Save<T>(T saveObject, string key)
     {
         string path = Application.persistentDataPath + "/saves/";
         Directory.CreateDirectory(path);
 
         BinaryFormatter formatter = new BinaryFormatter();
 
         using (FileStream fileStream = new FileStream(path + key + ".txt", FileMode.Create))
         {
             formatter.Serialize(fileStream, saveObject);
         }
     }
 
     public static T Load<T>(string key)
     {
         string path = Application.persistentDataPath + "/saves/";
 
         T returnValue = default(T);
 
         BinaryFormatter formatter = new BinaryFormatter();
 
         using (FileStream fileStream = new FileStream(path + key + ".txt", FileMode.Open))
         {
             returnValue =  (T)formatter.Deserialize(fileStream);
         }
 
         return returnValue;
     }
 
     public static bool SaveExists(string key)
     {
         string path = Application.persistentDataPath + "/saves/" + key + ".txt";
 
         return File.Exists(path);
     }
 }

and:

 public class GameEvents : MonoBehaviour
 {
     public static System.Action SaveInitiated;
 
 
     void Start()
     {
         
     }
 
     // Update is called once per frame
     void Update()
     {
         
     }
 
     public static void onSaveInitiated()
     {
         SaveInitiated?.Invoke();
     }
 }

I deleted the unneccesary code between, hope you can help me. I've read lots of Q&A's but they did not solve my problem. It works for other values: paraOyuncu and paraFiyat.