How to store Gizmo (grid) Cube positions in Vector3 list at game start?

I have written this script to create a grid of gizmo cubes:

 using UnityEngine;
 using System.Collections.Generic;
 
 public class Grid : MonoBehaviour {
 
     public int[] rows;
     //public int[] columns; //not integrated yet
 
     public float gizmoSize = 0.3f;
 
     public int width = 15;
     //public int length = 15; //I still don't know how to integrate this; for now width controls both width and length
 
     public List<Vector3> storedPositions;
 
     float storedPositionX = 0;
     float storedPositionZ = 0;
     int rowIndex = 0;
     //int columnIndex = 0;
 
     public float spacingX = 0.3f;
     public float spacingZ = 0.3f;
 
     public Color gizmoColor;

     void Start()
     {
         storedPositions = new List<Vector3>(); // this is where I want to store the gizmo cube positions
     }
     void OnDrawGizmosSelected()
     {
         rows = new int[width];
         //columns = new int[length];
 
         Gizmos.color = gizmoColor;
 
         foreach(int row in rows)
         {
             for(int x = 0; x < width; x++)
             {
                 Gizmos.DrawCube(new Vector3(transform.position.x + x * spacingX, transform.position.y, transform.position.z + storedPositionZ), new Vector3(gizmoSize, gizmoSize, gizmoSize));
             }
             if(rowIndex < rows.Length - 1)
             {
                 storedPositionZ += spacingZ;
                 rowIndex++;
             }
             else
             {
                 storedPositionZ = 0;
                 rowIndex = 0;
             }
         }
     }
 }

I tried to store the cube positions in the Vector3 List in the start method, but failed, because I don't know how to add every single position just once in the list, as the OnDrawGizmo Methods are being called every frame. Please help!