ga-cf7-submit.js

document.addEventListener( 'wpcf7mailsent', function( event ) {

    var form = event.target.querySelector( 'input[name="_wpcf7"]' ).getAttribute("value");

    if( parseInt( form ) == 4839 ) {

        gtag('event', 'form_submit', {
            event_category: 'Forms',
            event_action: 'Submit',
            event_label: 'Test form'
        });

    }

}, false );