Add a JS function in the Script file or in the page file

<script type="text/javascript">
function PrintGridData() {
var prtGrid = document.getElementById('<%=gvUserInfo.ClientID %>');
prtGrid.border = 0;
var prtwin = window.open('', 'PrintGridViewData', 
'left=100,top=100,width=1000,height=1000,
tollbar=0,scrollbars=1,status=0,resizable=1');
prtwin.document.write(prtGrid.outerHTML);
prtwin.document.close();
prtwin.focus();
prtwin.print();
prtwin.close();
}
</script>

call the javaScript in the Html onclick event

<input type="button" id="btnPrint" value="Print" onclick="PrintGridData()" />