Add a rowcommand event in your gridview like this:
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
onrowcommand="GridView1_RowCommand" CellPadding="4" ForeColor="#333333"GridLines="None">
Handle it in codebehind as;
protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
{
}