If you were to check in the Tools ... Developers Tool Kit ... and enter in the bottom input box:
TEXT_GV_REDEEM
and select Catalog ... and click Search ... you will see everywhere that is used ...
If you just want to change it for the ot_gv ... you could edit the language file for that Order Total module ...
Code:
define('MODULE_ORDER_TOTAL_GV_TEXT_ENTER_CODE', TEXT_GV_REDEEM);
and change what is being used in the define ...
Code:
define('MODULE_ORDER_TOTAL_GV_TEXT_ENTER_CODE', 'Fred was here!');
and now you are just changing that one thing ...