how sort filed gridview with call function in asp.net

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" AllowSorting="True" DataKeyNames="FUserId" OnSorting="GVAS_Sorting"  CssClass="GridViewStyle" DataSourceID="SqlDataSource1" Height="103px" Width="511px" ShowFooter="True" OnRowDataBound="GridView1_RowDataBound">
                          <AlternatingRowStyle CssClass="AltRowStyle" />
                        <Columns>
                         
                             <asp:TemplateField HeaderText="مجموع حقوق" SortExpression="SumWage">
                         <ItemTemplate>
                                    <asp:Label ID="lSumWage" Font-Underline="false" runat="server"
                                        Text='<%# SumWage() %>' />
                                </ItemTemplate>
                                <ItemStyle HorizontalAlign="Center" />
                                   <FooterTemplate>
                    <asp:Label ID="lbSumWage" runat="server" Text="Label"></asp:Label>
                </FooterTemplate>
                            </asp:TemplateField>
<asp:TemplateField HeaderText="نام کاربر">
                                <ItemTemplate>
                                    <asp:Label ID="fullNAME" Font-Underline="false" runat="server"
                                        Text='<%# TempFullName() %>' />
                                </ItemTemplate>
                                <ItemStyle HorizontalAlign="Center" />
                            </asp:TemplateField>

  </Columns>
</asp:GridView>
                    
                    <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ICTPortalDBConnectionString %>" SelectCommand="SELECT   FFamily,FName,FUserId From Tuser "></asp:SqlDataSource>

public string SumWage()
   {
      //

       return temp.retpricereq(u2,str).ToString();
   }