I've bought a physical copy of Witcher 3 about 5 years ago. I have been 14 years old then and got it as a gift so I didnt know much about how any of this stuff worked. Everything was working fine besides the fact I had a really old version of GOG Galaxy, I believe 1.6 or something. I wanted to buy the 2 expension packs but it wouldnt let me - it didnt register Witcher as connected to my account but I thought its because GOG is not updated. I did and after that I could not launch the game as it showed as unowned and wanted me to buy it again. I wrote to customer support and all they gave me was the code is used on a different account and theres nothing they can do. The account is of course mine, I have no way of logging into it as I made it 5 years ago and I cant prove I actually owning it besides having the physical copy with the used key, which in my opinion should be enough. Is there anything I can do? I can't afford buying the game again but really want to get the expansions. I dont want to pirate them as well as it just doesnt feel right.

If I understand your problem, you have created two different GOG accounts: you redeemed the game's key on your older GOG account and now you would like to link it to your newer account. I'm afraid this is not possible, because the game key can only be redeemed once and you are supposed to create only one GOG account.

If you remember at least the email address linked to your older account, I'm pretty sure you can retrieve your password and login: from there you will be able to buy the game's expansions.
Another solution might be contacting support and reporting your situation, asking them to remove your old account and provide you a new key for the game, but I'm not sure if they would want to help you in this way and you still have to prove that you are the owner of that old account.