Access Parent GridView Control in Child (Popup) Window and Populate with value

Populate Parent Window Grid view Control in Child Window.

Find the Parent Window Grid View Control Client-Id. if you don't how to find Client ID go through the View Source of page the after running.

Pass the Control Client - ID (the Client-ID like "GridView_ctl02_RemarksTextBox") to Child (Popup) through the Query String or any Other way(When the Control in GridView the Client-ID become change for each row) .
then in the popup window, you have the option to assign value to that control by the

opener.document.getElementById('control-Client-ID').value = 'New Value into that Control'