Why dont you use script?It's simple by using script..
Here is the sample try for it...
<script type="text/javascript" language=javascript>
function CheckNumeric()
{
var key;
if(navigator.appName == 'Microsoft Internet Explorer')
key = event.keyCode;
else
key = event.which
if ( !(key >= 48 && key <= 57) && key != 8 && key != 46 && key != 36 && key != 37)
{
event.returnValue = false;
}
}
</script>
[code]
//Other Gridview code
...
...
<itemtemplate>
<asp:TextBox id="txtPhone" text='<% # eval("Phone") %>' runat="server" onkeypress="CheckNumeric()"></asp:TextBox>
</itemtemplate>
[/code]
I hope now you are clear about the JavaScript validation on TextBox inside the Gridview