Add below code to your gridview. This should be your first column in grid:
<asp:GridView ...>
<Columns>
<asp:TemplateField>
<ItemTemplate>
<%# Container.DataItemIndex + 1 %>.
</ItemTemplate>
<ItemStyle HorizontalAlign="Right" />
</asp:TemplateField>
...
</Columns>
...
</asp:GridView>