Design a page with Gridview.
Open the page as popup page using
window.open()
In popup page, store the selected row value in state(Ex. Session)
Session["SelValue"] = "";
Close the popup using
window.close()
Using
opener.reload
you could reload the parent page
In parent page, get the value from session & assign the value as selected in dropdownlist.
dropdownlist.SelectedValue = Session["SelValue"].ToString()