I started with an idea of what that meant pretty abstractly in my head. But as part of this quest, and also as part of my own personal search to be more productive, I have been reading "Getting Things Done" again. My hope is that by rereading it I will get a better grasp of what the actual requirements of a good GTD todo list are.
Here is the secret then: It's pretty Simple. David Allen talks about all different kinds of systems from low-tech to high-tech. Of course because he wrote this book before the advent of the iPhone and the smartphone revolution that followed, what he means by high-tech is a palm pilot.
That's it, that is all he says you need for your lists.
By this criteria almost every todo list app out there qualifies to be used for GTD. All it needs to do is let you create lists and let you separate those lists into different sections. In this regard the app that most mirrors David's examples of the low-tech pieces of paper in a manilla file folder is the app Clear. And even it does more than that.
That is it, most apps can be GTD ready by the book's definition and the problem is that technological advancements in the last 10 years have changed a lot of our perceptions and the actual practice of GTD has morphed for a lot of people accordingly.
But that is a discussion I will save for tomorrow. For now know that whatever app You choose you can probably use it for GTD exactly the way that David Allen intended when he wrote the book. And that can help you remember to live better.