As noted above, if the pirated software does not represent a lost sale but is used for something that is a net benefit to the economy, it's a positive effect. And even if it does represent a lost sale, it still might be positive if revenue is redirected from the movie industry and instead to something that has a higher multiplier value like some more labor-intensive thing.
Clearly the assumption that it's all loss is ludicrous, the people who pirate software or whatever probably are mostly in segment of society that already use all their disposable income. If they couldn't pirate, they would either forego it or redirect their spending from something else. Given that the net savings rate of the US population, at least, is negative, it's not like people are hoarding cash while pirating stuff...
the GAO also noted that numerous experts told it that "there were positive effects [from piracy on the economy] and they should be assessed as well."
But it doesn't state any. Does anyone know what these positive effects might be?