There are enough horror stories regarding businesses built on Twitter &c to serve as cautionary tales, but Google and Gmail in particular are notoriously anti-developer. It is a nightmare to work with Gmail because, and I'm not even exaggerating, they actively prevent you from integrating with Gmail. Moreover, Google provides zero support and quite honestly hates you and everything about you.
No person considering a startup today should consider building it atop somebody else's platform. Even forgetting Google, it just doesn't make sense. You do not want to be beholden to a company that could not give less of a shit about you, and in all likelihood has never even heard about you.