Translating Next and Previous links in GP / WP Show Posts

Hi,
I have read a lot of support threads aboutt his topic, but I can't get any of the solutions to work.

Following David's and Tom's advice about using the Snippets plugin and then add a filter function:

add_filter( 'gettext', function( $text ) {
if ( 'Previous' === $text ) {
$text = 'Your new text';
}

if ( 'Next' === $text ) {
$text = 'Your new text';
}

return $text;
} );

just does not work.

Do I really need to use a translation plugin or open up the Po and Mo files and edit them?

Thanks (GP is a brilliant theme, so is GP Pro and WP Show Posts!)