Need to do few small changes your LoadData() method like
public void LoadData()
{
string sal= txtSal.Text;
var employees;
if(sal.Trim().Equals(string.Empty))
{
employees= employeeService.GetAllEmployees();
}
else
{
employees = employeeService.SearchEmployeeByType(sal);
}
gvEmployees.DataSource = employees;
gvEmployees.DataBind();
}
This should do your job !
Hope, it helps :)