Speaking on Google's Q4 2015 earnings call, CEO Sundar Pichai announced that at the end of 2015, Gmail had over one billion monthly active users. That's an absolutely massive number, but considering that we're talking about the scale of Google, it's actually the seventh product from the company to surpass that milestone.
The one billion monthly active user number is also shared by Google Search, Android, Google Maps, Chrome, YouTube and Google Play. Considering the synergies between these products it's not too surprising that Gmail finally made its way up to the top, especially since Google announced that the user figure was over 900 million as of Google I/O 2015.
Pichai also notes that the growth in active Gmail users is fueled particularly by mobile users, though we could've guessed that considering the absolutely massive number of Android phones in the wild.