Ппишешь в GridView:

... DataKeyNames="Id" ...
А потом в обработчике:
 
  void CustomersGridView_SelectedIndexChanged(Object sender, EventArgs e)
  {
   
    // Determine the index of the selected row.
    int index = CustomersGridView.SelectedIndex;
       
    // Display the primary key value of the selected row.
    Message.Text = "The primary key value of the selected row is " +
        CustomersGridView.DataKeys[index].Value.ToString() + ".";
   
  }
- - - - -
the truth is out there