We think you can trust the platform because we're fundamentally making Gmail better and more valuable to Google's users. There are a handful of successful businesses (like Streak and others) already built on top of gmail and have been for years. These venture backed companies already trust their business to be on top of Gmail. We're just making the process of building these apps waaaay easier.
We take security and performance really really seriously. Because our SDK helps you build Chrome extensions for Gmail, Google can (and has) shut down shady apps from the Chrome Webstore, so they don't have to shutdown the entire platform to get rid of a bad actors.
I absolutely commend you for what you are doing, I just wouldn't be willing as a developer to dip my toes in that particular water to spend the time developing something they could rip apart and break.
As a side note, the documentation page is broken on Mac Safari 8 on OSX Javascript error:
TypeError: undefined is not a function (evaluating 'reference.endsWith('()')')
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
Google will need to do something really intrusive to break their SDK