Type UnityEngine.GameObject is not marked as Serializable.

I'm having trouble with these classes. Is it not possible to serialize custom types? It was working fine before when I was testing with just the Class1 Class, but after finishing the script with all the Classes I need I can't seem to serialize these custom types. They all inherit from BaseClass which just holds some variables all the classes share, and are very similar like the examples below. I use a SaveData class that holds the variable info and constructor, and a SaveLoad class that does the serialization saving and loading- both of which had been working without a hitch 'til now. Error occurs while trying to save to path name.. If I need to provide more information please inform me. Thank you.

 using UnityEngine;
 using System.Collections;
 [System.Serializable]
 public class BaseClass{
     public string name;
     public int level;
     public int NRGcost;
 }

 [System.Serializable]
 public class Class1: BaseClass{
     public int bNumb;
     public float sTime;
     public GameObject[] alls;
     
     public void LevelUp(){
         level += 1;
         if(level == 2){
             NRGcost = 50;
             bNumb = 3;
             alls = new GameObject[bNumb];
             sTime = 8.0f;
         }
         if(level == 3){
             NRGcost = 150;
             bNumb = 4;
             alls = new GameObject[bNumb];
             sTime = 10.0f;
         }
     }
     
     public Class1(){
         NRGcost = 0;
         level = 1;
         bNumb = 2;
         alls = new GameObject[bNumb];
         sTime = 6.0f;
         name = "Class 1";
     }
 }
 [System.Serializable] //<<Gives error: SerializationException: Type UnityEngine.GameObject is not marked as Serializable.
 //>>taking it out gives error : Type Class2 is not marked as Serializable.
 //but nothing about Class1.
 public class Class2: BaseClass{
     public GameObject pow;
     public float pTime;
     public string pName;
     
     public void LevelUp(){
     }
     
     public Class2(){
         NRGcost = 0;
         level = 1;
         pTime = 7.0f;
         name = "Pow";
     }
 }