GTD stands for Getting Things Done - a popular time management methodology presented in a book by David Allen (subtitle: "The Art of Stress-Free Productivity"), which Wired Magazine (September 2007) described as a Cult of Hyperefficiency. A closely related approach is offered by the 43 folders website.
We might imagine that the value of Getting Things Done could easily be calculated by considering the cost (or opportunity cost) of Not Getting Things Done. But according to the old rhyme, the cost of Not Getting Things Done can escalate indefinitely.
For want of a nail the shoe was lost.
For want of a shoe the horse was lost.
For want of a horse the rider was lost.
For want of a rider the battle was lost.
For want of a battle the kingdom was lost.
And all for the want of a horseshoe nail.
So how would we estimate the true value of a nail? No doubt the blacksmith would argue the nail's case for being the critical success factor for the kingdom. But someone else might produce an equally plausible argument for something entirely different being the critical success factor.
So we might think that the value of Getting Things Done depends which things get done. But as Gary Wolf explains in his Wired review, Allen differs from a lot of earlier self-help gurus by skipping the philosophical reflection stage. "Instead, he likes to describe his system as a 'bottom up' approach, by which he means that life's values emerge from its tiniest component actions, rather than a top-down approach that starts with deep thought."
So the purpose of your actions is whatever you find yourself doing, emerging from a series of micro-decisions that are triggered by a large number of events (such as incoming email). In other words, POSIWID. The aggregate value of this activity is determined by its overall coherence, rather than its correspondence (alignment) with any externally given goals and values.
In their paper Getting Things Done: The Science behind Stress-Free Productivity (pdf), Francis Heylighen and Clément Vidal describe Getting Things Done as a praxeology - in other words, a theory of practical action. They point out that it is intrinsically difficult to compare the productivity of people using GTD with that of people using different methods.
"The reason is that because GTD does not embrace explicit priorities or optimization criteria, there is no obvious standard by which to measure expected productivity enhancements. A simpler approach may be subjective evaluation: how satisfied with their work are GTD users compared to users of other methods? However, this will still teach us little about precisely how and why GTD is supposed to work."
Furthermore, because there are no obvious standards, it is not clear what other methods it would be appropriate to compare with GTD. How about the Rule of Saint Benedict, for example? (The Rule of Saint Benedict was of course originally intended to govern a community of monks, although some people think it can have secular applications. Whereas GTD appears to be aimed at individuals rather than organizations.)
Given that GTD is billed as a way of reducing stress, perhaps it would make more sense to assess its value in those terms - in other words, evaluate it not as a praxeology but as a New Age therapy. So we should be comparing it not with the Rule of Saint Benedict but with Scientology.