It's a simple tag you can add to links.
gtag('event', 'click', {event_category: 'Mobile Button', event_label: 'Contact'});
The two things highlighted in yellow are the things you can customize. The first is the event category and the second is the event label.
So the full link for the mobile button would look like this:
<a class="btn btn-block btn-primary" href="/contact" onclick="gtag('event', 'click', {event_category: 'Mobile Button', event_label: 'Contact'});">Contact Us</a>
