UnityEditor.GameViewSizeGroup.IsCustomSize C# (CSharp) Method

        public bool IsCustomSize(int index)
        {
            if (index < this.m_Builtin.Count)
            {
                return false;
            }
            return true;
        }

 public bool IsModificationAllowed(int index)
 {
     return(m_GameViewSizeGroup.IsCustomSize(index)); // builtin sizes cannot be modified
 }