Customizer JS API get value of customizer field

You need to do it the same way you’d get options, shortcodes, or any other WordPress value into JavaScript: You need to get them with PHP and pass them to the script with wp_localize_script():

wp_enqueue_script( 'my-script', asset_path( 'scripts/plugins.js' ), [], null, true );
wp_localize_script(
    'my-script',
    'myScript',
    [
        'galleryTransition' => get_theme_option( 'gallery_lightgallery_transition' ),
    ]
);

Just change my-script and myScript to something specific to your project.

Now you can access the value in your script via myScript.galleryTransition.