How to get GridView values from asp:BoundField?

You can try the foreach loop to accomplish your requirement which binds the GridViewRow class to your GridView Source.

foreach (GridViewRow gr in GridView1.Rows)
{         
    string cell_1_Value = gr.Cells[0].Text; 
    string cell_2_Value = gr.Cells[1].Text; 
}

As you know, the Grid View has multiple rows. So you can get the values from any of the rows.

Getting it using the Data Keys:

var ponumber = GridView1.DataKeys[rowIndex].Values[0].ToString();
var sitename = GridView1.DataKeys[rowIndex].Values[1].ToString();

Getting it by cells:

// easiest way, but not the most recommended.
var ponumber = GridView1.Rows[rowIndex].Cells[0].Text; // try 0 or 1

you can also get the data in the Row Data Bound Event:

protected void GridView1_RowDataBound(Object sender, GridViewRowEventArgs e)
{
     if(e.Row.RowType == DataControlRowType.DataRow)
     {
       var ponumber = GridView1.DataKeys[e.Row.RowIndex].Values[0].ToString();
       var sitename = GridView1.DataKeys[e.Row.RowIndex].Values[1].ToString();
     }
}

foreach (GridViewRow gr in userGrid.Rows)
{
    CheckBox check = (CheckBox)gr.FindControl("chkSelect");
    if (check.Checked == true)
    {
        string order = userGrid.Rows[gr.RowIndex].Cells[3].Text;
        gl.updatetracking (order);
    }
}