Does anyone know if there's a way of downloading like, a receipt or something for games purchased from GOG that would be e.g. legally admissible in court?

Depends on the proof you want but if you save or copy your receipt from email you get after your purchase you can use that or if all else fails take a screenshot of your order history and send it to GOG.

Privacy and Settings --> Order History --> View Receipt

Or, you have email receipts sent to you when you complete a purchase.

Ahh, neat! I must admit I never would have thought to look under Privacy and Settings for Order History!

A shame I need to dump them one by one, but at least they are there and in a way that includes order numbers and identification.

Alas with e-mail, a lot of the earlier games receipts were on an e-mail server that no longer exists and are probably lost with it, so it's good that this exists.

Thanks for that!

Out of curiosity, for what reason would you need a proof like that?

Your honor, as this receipt clearly states, I was buying DRM free games at the time the murders occurred.

How is this statement in any way helpful to the OP?

By GDPR you can demand a log of every interaction with and through GOG, which includes purchases, chat, forum posts... Somebody had done this iirc and they got a huge JSON file (which is the most common format for that, Google does the same) which basically contains everything.

About the "legally admissible", that's of course a problem. Every digital file in your possession could be tinkered with, and if GOG weren't around any more to back up your claims it'd be a weak proof, unless there were some digital signatures and hashes attached.