When you are using Google Analytics to follow consumers on your website, you also want the consumer's turnover correctly registered in Analytics.
The easiest way to do this is by placing the Google Analytics tracking on the thank-you page, also known as the return URL. To get your turnover included in Analytics too, you would probably have to set up Ecommerce Tracking. Google has written an article including instructions on how to set this up.
Since it is possible that not all consumers return to your website after payment (for instance because they close the screen), you can also choose to submit the information via the Analytics Measurement Protocol. In that case, your server submits the information directly to Google Analytics; the consumer doesn't have to be on your website for this. You can do this at the moment we call the webhook for the payment.
Unfortunately, Mollie cannot submit information to Google Analytics on your behalf.