The gettext WordPress filter can be used to translate internationalised I18n text – that is, __() or _e(), without using complicated PO and MO files. If you need to change the text for an _x() string you will need to use the gettext_with_context filter.
In this example we’re using the gettext filter to change the Gravity Forms ‘This field is required.’ string to lower-case ‘this field is required’.
To use, copy into your theme’s functions.php file below the opening <?php line
add_filter( 'gettext', 'itsg_translate_text', 20, 3 );
function itsg_translate_text( $translated_text, $text, $domain ) {
if ( "This field is required." == $translated_text )
$translated_text = __( 'this field is required.');
return $translated_text;
}