Using an outline for GTD is the way to go for sure! It allows for unlimited depth of sub-projects and actions, which is a key to breaking down your projects into small actionable steps!
We’ve taken that concept and built a task management app on top of an outline. It gives you all the benefits of an outline but adds in automation that is specific to task management and GTD.
For example, instead of having to tag all of your next actions, we automatically take the next action in each project and build a dynamic list that is always up to date with all your next actions.
We would love for you to take a look at it and let us know what you think! It’s free at the basic level http://GTDNext.com