Pass some identification parameter(s) to popup window as GET parameter (in URL). When popup opens, load selected row from DB using those identification parameter(s).
Tricky part is to get gridview refresh itself after popup is closed. You can do this by JavaScript (a quick, untested example):
Parent page
Code:
<script type="text/javascript">
</script>
Popup
Code:
<script type="text/javascript">
</script>
<asp:Button ID="CloseButton" runat="Server" Text="Close" OnClientClick="CloseMe(); return false;">