MailSlayer integrates with GMail by leveraging a browser extension (currently available for Chrome and soon to be available for Firefox) that injects JS into GMail at runtime.

Google doesn't have an API for GMail, and is on record saying that they have no plans to release one. GMail's JS itself is extremely obfuscated (Google uses their Closure compiler to compress it before pushing to production). Interfacing with GMail has definitely been a challenge, but after some sleepless nights and a bit of luck we have a stable and flexible platform. This is one area where we can offer major time-saving value to developers.