Using your example of GTD, where does GTD appear in your six horizons?
If the answer is 'nowhere', then I'd say that's an issue.
In my case, GTD is one of my Areas of Focus.
I accumulate notes at each node of my six horizons.
I have a folder structure on my computer like
GTD/
GTD/50K Purpose and Principles/
GTD/40K Visions/
GTD/30K Goals/
GTD/20K Areas of Focus/
GTD/20K Areas of Focus/GTD/
GTD/20K Areas of Focus/Health/
...
GTD/10K Project Support Material/
GTD/10K Project Support Material/Brother - Buy birthday present/
GTD/10K Project Support Material/Hand - Rehabilitate injury/
...
I treat each of the 20K+ nodes as parents of Next Projects, in the same way that projects are the parents of Next Actions. So each of these nodes can have support material in the folder. When a project ends, I go back to the parent to create a Next Project, which triggers a review of the support material, as one would do when creating a next action.
I have text documents with names like 'About Projects', where I gather my tips and tricks for running projects.