Let's say, I purchased a game here on 2010, now have it through a collection as a physical disk/whatever, is it possible to give that game, now kinda useless on your account, to someone who doesn't have it?

Nope.

Damn. No exchanging either? Meaning, you have what I want, I have what you want, let's trade? Noobie here, hello.

Also nope. Since these games have no DRM, there is no way for GOG to guarantee that you didn't keep a copy of the game that gave away/traded, so all that is disallowed.


EDIT - noobie? Your join date is 5 years ago?

Noobie as in trading/gifting and all around whatnot ; pro as buying and keeping a keen eye on possible sales.

If you have bought a game and never ever downloaded it or any of its extra bonus goodies, you can get a refund for it within a short window of time after purchase. I forget the exact policy but it's something like 30 days, I'll defer to someone else for validation or a link to the policy. If the game has been downloaded however, then you own it. There is another policy for if a game doesn't work, in which case GOG support will assist in getting it to work. Usually (in my experience) they are very good at this. If the game wont work with their assistance and it is within a certain timeframe of purchase, they may issue a refund.

If you've bought a game and had it on your shelf for months/years however then I don't think there is any way to receive a refund. In some cases it is possible they may consider converting a game on one's shelf that has not been ever downloaded into a giftable game code, but they do not advertise or promise this so it is never guaranteed, and it is also less likely the longer one has owned a game.

The best advice is to only purchase games here for yourself to put in your own library if you fully intend to keep and play them. If you are ever unsure about a purchase, or simply are collecting games as they are a good deal on sale but aren't certain you'll play them right away or ever, then it is perhaps better to purchase the game as a gift using your own email address. Then you receive a gift code that does not ever expire, and you can use it yourself at a later date if you wish to redeem the game to your own account, or you can gift the code away to a friend or in a community giveaway.

Once a code is redeemed to an account however, it is locked to that account unless it qualifies for the return policies mentioned above though. The best thing for one to do who is ever unsure if they'll play a game, is to always purchase them as gift codes keeping in mind that some games such as The Witcher 3 and its expansions are not able to be purchased as gift codes.

Hey, I joined 3 years ago and knew next to nothing about gifting or the forums for some time.
But, basically, he's right. Were GOG to allow this, you could get legions of potential scammers who download the game and then say that they don't want it anymore.
Now, had you bought the game RECENTLY, that's a different subject. The staff might be understanding enough to refund you or convert it to a gift code. This happened to me with the Pinata promo. I didn't exactly understand how it worked, and bought 3 mystery games, all of which, upon seeing, I knew I was never going to play. I immediately (maybe 5 minutes after the purchase?) contacted the staff about it, and they were kind enough to give me a refund. When the mystery gift promo came around, I was somehow stupid enough to do the exact same thing again, and this time, the staff converted it to gift keys. After that, I learned my lesson, and all mystery purchases I made, I made sure they were GIFTS. I simply gifted it to my own email, saw what the games were, and if I wanted them, I kept them. If not, well, that's how I got started on these forums! Traded 'em off.
Long story short, I think what mattered in both of these cases was how quickly I responded (who the heck downloads 3 games in the span of 5 minutes while writing a support note at the same time?). If the purchase is recent, you've got a shot. If it's old, forget about it.

Treat it as a back-up. Always good to have a digital copy in case something happens to the physical one.

This could work, but only for game installers that haven't been downloaded yet from gog's servers (yes, gog keeps track of this).
Therefore please vote for this Feature Wishlist:
https://www.gog.com/wishlist/site/lets_us_gift_away_games_from_our_library_that_we_havent_downloaded_yet