Quick Actions can increase the number of users who decide to attend any webinar or event you plan to host. Marketo should support Google 'Quick Actions'. However, currently Gmail has quick actions for activities like RSVP to a meeting, View Video, open any document in Google Docs folders.
Quick Action in Gmail will certainly reduce the open rate slightly. I suggest you add your CTA right next to your email subject line. Trying A/B testing for your email campaigns and see what works out best for you.