Decimal Validation for TextBox in TemplateField of ASP.Net GridView

You can use MaskEditExtender

<asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
        <asp:BoundField DataField = "Item" />
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:TextBox ID="txtAmount" runat="server" />
                    <cc1:MaskedEditExtender ID="MaskedEditExtender1" runat="server" TargetControlID="txtAmount"
                        Mask="9{9}.99" MaskType="Number" InputDirection="RightToLeft" />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>