"Offline Gmail" is no longer a lab feature - it's now part of the official Gmail offering:
Almost a year ago, we launched Offline Gmail in Gmail Labs. By installing Offline Gmail, you're able to use the normal Gmail interface to read and write mail, search, and organize, even when there's no internet connection. And Flaky Connection mode speeds up Gmail when your connection is slow or unreliable.
Tip: To get started with "Offline Gmail" and Mailplane, please check out the how to article.