I think it's pretty safe to say that most people who are not using some kind of stand-alone todo list are using their email inboxes as their de facto todo list. While that clearly works for some people, it's not kosher in the Getting Things Done universe.
If you're a Gmail user and you're finding yourself overwhelmed with trying to keep track of things in your inbox, but don't really want yet another place to check on things, consider trying GTDInbox. GTDInbox is a Firefox Add-on that adds todo list functionality to Gmail that is far more functional than the pathetic Tasks functionality that is built in to Gmail.
The functionality of GTDInbox is impressive. It uses a set of pre-defined labels to track the tasks that are sourced from email messages. You can create new tasks that are not linked to email messages, and track them all in a very Getting Things Done compatible environment.