You can try this code in your WebSite :
<![CDATA[
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript">
function openForm()
{
window.open("https://accounts.google.com/ServiceLogin?service=mail&passive=true&rm=false&continue=https://mail.google.com/mail/&ss=1&scc=1<mpl=default<mplcache=2&hl=en");
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:button id="Button1" runat="server" text="Google Login" onclientclick="openForm();" xmlns:asp="#unknown" />
</div>
</form>
</body>
</html>
I hope it will help you. :) --MKB