As i understand ButtonField does not support OnClientClick property.
Alternate solution would be to use a TemplateField to define ASP Button and call its Onclick event as follows.
<asp:gridview id="GridView1" runat="server" width="100%" xmlns:asp="#unknown">
DataKeyNames="NoticeId" onrowdatabound="GridView1_RowDataBound"
onrowdeleting="GridView1_RowDeleting" AutoGenerateEditButton="True"
onrowediting="GridView1_RowEditing">
<columns>
<asp:templatefield>
<itemtemplate>
<asp:button id="Button1" runat="server" text="Delete" commandname="Delete" onclientclick="return ConfirmDeletion();" />
</itemtemplate>
</asp:templatefield></columns>
</asp:gridview>
//Script in header tag
<script>
function ConfirmDeletion()
{
return (confirm("Are you sure you want to delete the record?"));
}
<script>