Forcing a term/taxonomy on all posts of CPT

I have a CPT – ‘Events’ with 400+ posts [with no terms in the custom taxonomy (tax_category) selected] in them, as I recently imported those posts.

Now, I’ve created a URL structure where I need to have some term of tax_category assigned to each post. So, is there any way with which I can assign a term from tax_category (say ‘New Gigs’) to all posts of the Events cpt?

While searching I also found the following code, but am confused of how can I modify it suit my need or it even is the solution am looking for? –

function mfields_set_default_object_terms( $post_id, $post ) {
        if ( 'publish' === $post->post_status ) {
            $defaults = array(
                'post_tag' => array( 'taco', 'banana' ),
                'monkey-faces' => array( 'see-no-evil' ),
                );
            $taxonomies = get_object_taxonomies( $post->post_type );
            foreach ( (array) $taxonomies as $taxonomy ) {
                $terms = wp_get_post_terms( $post_id, $taxonomy );
                if ( empty( $terms ) && array_key_exists( $taxonomy, $defaults ) ) {
                    wp_set_object_terms( $post_id, $defaults[$taxonomy], $taxonomy );
                }
            }
        }
    }
    add_action( 'save_post', 'mfields_set_default_object_terms', 100, 2 );

Code Source – https://wordpress.mfields.org/2010/set-default-terms-for-your-custom-taxonomies-in-wordpress-3-0/