Understanding how a system like GTD works is the theory. You have to put the theory into practice to gain any benefit. Typically that means finding a tool to store all your tasks and setting it up in a way so you can make use of the system.
Creativity with it’s winding and meandering journeys and productivity with its straight line efficiency don’t always get along. What you do to improve one seems to reduce how well you can complete the other.
Whether you think you can or you think you can’t—you’re right