Validate whether GridView has rows or is Empty using Custom Validator in ASP.Net

Try this

<asp:GridView ID="GridView1" runat="server">
    </asp:GridView>
    <asp:CustomValidator ID="CustomValidator2" runat="server" 
        ClientValidationFunction="gvPostDatedChequeValidation" ErrorMessage = "GridView has no rows."></asp:CustomValidator>
    <asp:Button ID="Button1" runat="server" Text="Button" />
    <script language="javascript" type="text/javascript">
        function gvPostDatedChequeValidation(source, args) {
            var grid = document.getElementById("<%=GridView1.ClientID%>");
            args.IsValid = grid != null;
        }
    </script>

Now you can easily attached Validator callout to the Custom Validator and use it