Game disappear from GOG Galaxy library?
Check GOG library page in Your browser.In my case games sometimes disappear from Galaxy (some bug I think),couple of restarts usually fix the problem.

I checked everywhere, its just gone. I still have old files on my computer, so i could proof i had it,but in my lybrary its just gone an i need to buy it again if i want to play it.

Where did your friend get the gift code? If he didn't buy directly from GOG but from some shady reseller like G2A chances are the code was originally bought fraudulently with hacked Paypal account or stolen credit card. In those cases the gift code (and ownership of the game) are revoked.

Real proof would be the order confirmation email from GOG when you bought the game. If you bought it from GOG, it shouldn't disappear. If for some freaky reason it is gone, show GOG your order confirmation, order number and everything.

Like i said, it was a gift from years ago. No chance to find those. Dont know. he sent it to me as a gift and it was for years in my library, now its gone. Dont know what to do, i only know i will never buy a game over GOG again if this can happen and i dont even get help from the support.

Gifts show up under https://www.gog.com/account/settings/orders with Gifted From (username) and Price: Free (Gift).

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I've never had any game disappear on me. Even if GOG removed a game from the store, one that you have bought, it would still be in your library. Your order history should have everything. If not, then I say you misremember the name of the game and/or which store the game was for.