You need to add an event in your gridview
<asp:gridview id="gvKategori" datasourceid="SqlDataSource1" runat="server" allowpaging="True" autogeneratedeletebutton="True" autogenerateeditbutton="True" autogeneratecolumns="False" borderstyle="Solid" borderwidth="1px" pagesize="5" OnRowDeleting="GridView1_RowDeleting"></asp:gridview>
and implement your event like this
protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
}