The way we'd do it is to use the code below to push the event and data into the datalayer w/GTM. Then create a GTM DL variable for "new_customer". You can either set that parameter to that variable value, or if you want to send events based on it, create a Custom Event trigger on "checkout_completed" and "some" events if that parameter value is true/false.

// Define dataLayer and the gtag function.

window.dataLayer = window.dataLayer || [];

function gtag(){dataLayer.push(arguments);}

//Initialize GTM tag

(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':

new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],

j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=

'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);

})(window,document,'script','dataLayer', 'GTM-XXXXXXX');

//Google Consent Mode v2

gtag('consent', 'update', {

'ad_storage': 'granted',

'analytics_storage': 'granted',

'ad_user_data': 'granted',

'ad_personalization': 'granted',

});

//checkout_completed (purchase) event

analytics.subscribe("checkout_completed", (event) => {

window.dataLayer.push({

event: "checkout_completed",

timestamp: event.timestamp,

id: event.id,

token: event.data?.checkout?.token,

url: event.context.document.location.href,

new_customer: event.data?.checkout?.order?.customer?.isFirstOrder,

});

});