I just logged in to my GOG account in what seems like forever and noticed that my games library is completely empty. Admittedly, I have used GOG very little over the years and cannot really remember what I have purchased but I¨m positive that I have owned some free or really cheap games such as the original Fallouts, the first Witcher game (which I own physically and activated through GOG years later), Beneath A Steel Sky and probably some other old DOS game.

I have looked around through filters etc and can confirm that I do not own a single game ATM. Any ideas?

Registered: Jan 2023

His profile also says "User since Sept 2022" though. Something is seriously broken there...

That is indeed strange.

I know for a fact that I have used GOG years ago, certainly before september 2022. I had a username and password already saved to my Google account.

"User since September 2022"
"Registered: Jan 2023"

Did you sign up with Facebook by any chance? Regardless, you need to open a support ticket. You can try opening the refresh links one at a time in a web browser, but I wouldn't expect any change.
• https://www.gog.com/account/refresh
• https://www.gog.com/user/refresh

Okay, so it's probably worth you checking that your account ID saved to Google is your main account and not an alt. You will need to contact support. You should have email confirmations of any orders you made through the website (including redeeming free games), which will have been sent to the email account that you had registered with GoG. So, there should be a body of evidence which you can present to support if your account has lost its games.