Yeah I'd pretty much have to agree mate. As cool as it would be to use the weclock-leung system, it'd not be economically viable. Lets assume there's 1000 GOG customers (probably way more than that) and the free birthday game is a $6 one, that would average to 16 games given away daily at a total cost of $96/day. Sure the numbers would be reduced substantially if it required forum participation as well as financial participation but frankly a business, no matter how cool, would prefer the latter.

At the same time though, I don't see anyone being a viable member of the community, not buying a game, at least not eventually.
Yes, purchase is necessary to keep a company alive, but at least by supporting the site via forum activity, you prove that you care for the site. I'm not saying "nobody should buy a game." I'm saying that you should need to have X number of games to qualify, because what if you are simply only interested in one game? Forcing people to buy a game they don't want, regardless of their forum activity, is wrong. Especially if it's supposed to be something like a birthday present.
Despite how rep can be abused (only by making multiple accounts), I simply don't think it's going to happen. I'm aware of how someone could abuse the rep system, I wrote the damn book on the rep system. I could easily abuse it myself. Remember the only reason I care about Max rep, is the free reward you get, without having to purchase a game in the first place. :D
So, to sum it up:
1. The Rep system can be abused, but it's abuse can be easily detected, by GOG staff and by regulars here. Abuse will not be tolerated.
2. Not only has GOG had contests in the past that don't require a purchase, they have one going on right now that everybody who posts in the forum is participating in, to which level is up to you.
3. If I chose to abuse the Rep system, as you say, I'd have max rep right now, and the free reward.
I might be willing to compromise, say if you bought one game and had a certain amount of rep (as rep is the only real counter of positive forum participation), then I could see that. But to say anyone who's bought 1 game, can get a free game on their birthday, or anyone who's bought a number of games can get a free game, is perhaps a little farfetched.
You'll see people who don't care about the community, people who bought games and got out, getting these rewards. I would rather see people who care about the community as well as their business being rewarded.

You'll see people who don't care about the community, people who bought games and got out, getting these rewards.
And why shouldn't they be rewarded? They are paying customers afterall. They are the most important people for a company (in terms of those below the clients and investors). A person simply posting on the forum and collecting rep, while not purchasing games, is in no way more important to a company than those who actively increase the companies profit.
Again, look at brick and mortar stores, or other online services, and you'll see it's the frequent customers that are rewarded. Surely you don't expect to go to a store once, buy one product, then expect the same discounts as someone who has made frequent purchases; earning enough points on a card to grant them X amount off their purchases. So why would you expect the same in this form of business?
For that matter, why do you really care so much about not getting access to a particular perk simply because you haven't found X number of games you want to purchase? I really get the impression that you don't really understand the point of such curtesies.
Besides, if you didn't find enough games you wanted to purchase, why would you then want one of those games for free? If you didn't want it enough to purchase it; then you're just trying to get it free for the sake of it being free. And that's not really the sort of people a company should be rewarding. Sorry.
Despite how rep can be abused (only by making multiple accounts)
Sorry to burst your bubble but that's not the only way to abuse the system. And I urge you not to try and derail this thread searching for other ways to abuse it. Someone is bound to take advantage of such information if made public.
Remember the only reason I care about Max rep, is the free reward you get, without having to purchase a game in the first place. :D
I hate to say it, but you come across as way too selfish with comments like that =/.