functions.php

<?php

function add_gb_script() {
    wp_enqueue_script(
        'gb-script',
        get_stylesheet_directory_uri() . '/gb_script.js',
        array( 'wp-i18n', 'wp-element', 'wp-blocks', 'wp-components', 'wp-api'  )
    );
}
add_action( 'enqueue_block_editor_assets', 'add_gb_script' );

gb_sript.js

wp.blocks.registerBlockStyle( 'core/button', { name: 'custom-btn1', label: 'Custom 1', isDefault: true });