The gifting I have in mind is the gifting in the vein of trees: you get their gifted oxygen as a matter of fact, but you don't reward the most productive tree with most of your gifted CO2 -- you give it freely for every tree and plant to take.

I went through your Intro to the Philosophy of Contributionism, but, as far as I get it, it's far from gifting -- the contributors are still rewarded. Even if you base a credit system on a "carrot," you still, by very definition, create a value hierarchy. And every value hierarchy can be gamed. What will be considered a valuable contribution? Who will decide that? Not to mention that creating perks is basically creating "start" restrictions (or default handicaps).

Yes, you address those issues, but many of those solutions have been tried, and, frankly, they don't work in the end due to populism and agents who manipulate the hierarchy structure by skillful persuasion. Let's take open-source projects you mentioned for example: the amount of internal politics involved in only such matters as whose commit or whose design ideas get accepted is still enormous as individual egos play a huge role even there (not to mention that those projects live from rather huge funds, too, so there's a lot of money involved at the top levels).

Therefore, I call "contribution to society" an euphemism -- strictly speaking, every individual contributes to society by the fact of their very existence, and that simply cannot be measured in any meaningful or objective way.