I found convincing indications on where theoretical imputs on chapter III of the encyclical came from.
I read in an interview to the german catholic philosopher Otto Kallscheuer (in the Italian opinion newspaper Il Foglio, of 7-9-09), that "the theme of gift and its gratuitousness is fruit of the teem work realized in the Pontifical Academy of Social Sciences by Stefano Zamagni, speaker for the economy of brotherhood, by Alain Caillé, critic of the utilitarian paradigm and founder of the MAUSS (Mouvement anti utilitariste dans les sciences sociales), by the Nober prizes Joseph Stiglitz and Amartya Sen, even though the latter was not present".
It sounds convincing because Alain Caillé is author of a book on gift, Le tiers paradigme, in whic he declairs himself directly inspired by Marcell Mauss (to the point of makng anacrostic of his name).
However, in the pages Benedict signed there is an insight that goes beyond what I found in Caillé's book. Or at least that is what I read in it with the interpretation I gave of gratuitousness.
The ground inspiration might simply be the archetype of the good business man, as we can find in an old Hollywood movie. In Sabrina, Humphrey Bogart, businessman of great consequence, is organizing the marriage of his brother to launch a new enterprize. Asked why he does it, not for money nor for poweer, which he already has, he answers that he does it for its own sake, no other reason than the pleasure of building things and creating wealth. The answer, with its purely gratuitous motivations, sounds good to me, and it remains valid also after that he leaves behind everything for a more perfect gift, the gracious love of the girl Sabrina (a splendid Audrey Hepburn).
Who knows, Joseph Ratzinger might even have seen it.