I concur 100% with CanSpice.

I saw the email from a contact, clicked through and saw that your app only asks for "view contacts" so I though I'd be able to choose to whom I'd send an email or at least have some kind of confirmation before all of a sudden "BAM" you send thousands of emails to everyone I've emailed in the past 5 years.

That's really bad. You have to improve the UX/UI so that there's a bit of granularity and more transparency otherwise, you're going to lose your Google API access... or simply disappear as everyone marks your emails as spam.