I use taskwarrior and only some methods of GTD (some tasks are assigned a lot of metadata if they are quite simple). Nevertheless here are my ideas:
A simple way to display all the task without a project would be a filter like
task list pro:
task new limit:20 pro:
if you prefer only the 20 newest tasks. Make yourself familiar with custom reports, then you can easily create a report called "review" that shows you the items you recently added.
Another idea would be to create an alias on your shell, that expands to "task add pro:inbox". Then you can add task using your alias, the added items are assigned to the "project" inbox which you can then filter on. I would not recommend tags for this since you would remove the tag after assigning the task to another project.