Right - an embedded webview with some window dressing. But it seems like they don't want to handle it, as they've made clear. So why not shell out a few bucks for one that's already made?
But this person might land a job at Google, like the Sparrow folks... http://sparrowmailapp.com/
That's not an insult, it's supremely awesome that this is even possible and that this dev made it happen. I'm using it now. But there are a few of these types of things for Google Music, there's a similar Chromebook app.