All right, first things first: One of the most frustrating flaws with Android's current gesture setup is the way the universal Back gesture — where you swipe in from the left side of the screen — overlaps with other actions already present throughout the operating system.
The most common conflict is with the gesture for opening a drawer-style menu within an app, like what you see within Gmail or Google Drive. Google created an awkward mechanism for differentiating between swipes meant for going back and swipes meant for opening an app menu, but it's clunky, inconsistent, and generally just too unpredictable to rely on.