GTD ... established circa 1939!
The term of "Getting things done" has been used as the, or as part of, many book titles. Douglas Johnston at A million monkeys typing had mentionned coming across a copy of Getting Things Done by Edwin C. Bliss. I could not resist (trust an Historian on that one!) having a look at the catalogue of at both the British Library in London and the Library of Congress in Washington, in search of the earlier use of the expression and acronyme.
The ealier use of the Getting Things Done as a book title is :Wilson, Everett Broomall. Getting things done in business. New York : McGraw-Hill, 1937.
The acronyme in question was used as the title of a book published by one Percy Redfern Creed before David Allen was born:
"Creed, Percy R. (), b. 1874.
G. T. D.,
[Boston, Mass., The author, c1939]
118 p. pl., ports. 23 cm."
The book in question was republished after the Second World War, this time as Getting Things Done:
Creed, Percy R. (Percy Redfern), b. 1874.
Getting things done, by Percy R. Creed ...
Boston [The Merrymount Press] 1946.
xvii p. 1 l., 95 p., 1 l. port. 24 cm.