Gridview is rendered as Table in WebBrowser. so you can access the controls using table rows.
like
<script type="text/javascript">
var table = document.getElementById("GridView1");
if (table.rows.length > 0) {
for (var i = 1; i < table.rows.length; i++)
{
var elements= table.rows[i].getElementsByTagName("input");
}
}
</script>