Without property rights, I'm not sure what if anything -- except services, a.k.a. labor -- can be consistently monetized in a free market.

In other words, in a world without property rights, the only thing of (monetary) value is labor.

And if memory serves, the labor theory of value is the foundation of Marxian economics; see http://en.wikipedia.org/wiki/Labor_theory_of_value.

Consequently, it's not clear to me how it's possible even to have a free market in intellectual work product, as opposed to a free market in labor, without some kind of legally-enforceable rights in intellectual property.

Bias disclosure: I'm an IP lawyer by background.

EDIT: The GPL couldn't exist without IP rights, that is, the right (under copyright law) to haul others into court to force them to comply with the GPL terms or to fine them for failing to do so.