try this sample, add jquery reference to it.
code behind:
DataTable dt = new DataTable();
dt.Columns.Add("id");
dt.Columns.Add("name");
dt.Columns.Add("value");
dt.Rows.Add(1, "priya", "test");
dt.Rows.Add(1, "vinu", "test");
dt.Rows.Add(2, "ram", "test");
dt.Rows.Add(3, "set", "test");
dt.Rows.Add(3, "set", "test");
dt.Rows.Add(4, "set", "test");
GridView1.DataSource = dt;
GridView1.DataBind();
<script>
$(function () {
var clientId = '<%= GridView1.ClientID%>'
var tds = $('#' + clientId + ' tr ').find('td:nth(0)');
var temp = []; j = 0;
for (var i = 1; i <= tds.length; i++) {
debugger;
var prev = $(tds[i - 1]);
var curr = $( tds[i]);
if(j==0)
temp.push(prev);
if (prev.text() == curr.text()) {
temp.push(curr);
j++;
}
else {
$(temp[0]).attr('rowspan', temp.length );
for (var k = 1; k < temp.length; k++) {
temp[k].remove();
}
temp = [];
j = 0;
}
}
});
</script>