iterate through ALL rows in a GridView

Answer 1

Look at your logic - you only have the one checkbox. You're unchecking and checking the same control in the employee loop. Does each grid row have a checkbox that should be selected based on the condition the id exists in the employee list?

 foreach (GridViewRow row in gv.Rows)
    {
        Label Id = row.FindControl("lblId") as Label;
        var result = Employee.GetEmployeeById(Id.Text);
        if (result.Count > 0)
        {
            CheckBox chkBox = row.FindControl("chkSelected") as CheckBox;
            if (chkBox != null)
            {
                chkBox.Checked = result.Any(x => x.Id.ToString() == Id.Text);

            }
        }

    }