The title could be a bit misleading, GTDGmail is a firefox extension that adds additional functionality to your gmail account. Getting Things Done is a productivity concept that aids you in organising your tasks: “a person needs to move tasks out of their mind and get them recorded somewhere. That way, the mind is freed from the job of remembering everything that needs to be done, and can concentrate fully on actually performing those tasks”
GTDGmail adds several new features to your gmail account like advanced labels, a outstanding tasks list, the ability to save specialised searches and to send new tasks to your account. I think some screenshots might help understand the concept, let us take a look at some screenshots:

Labels are structured into Statuses, Contexts, Projects and References. Status simply describes the current status of the task, is it finished or should it be the next action that you perform ? Context is the context that you have to be in to complete the task. A task that would require a computer for instance could have either the Desk, Home or Office context associated with.
I suggest you install the application and play around with it a little bit. It might look confusing but once you actually use it you will soon realise that this is a great time safer.

Gmail Tasks Can Now Be Emailed
Gspace Firefox Extension
Gmail Backup
Add functionality to Gmail with Gmail Labs