Pheace: Yeah, kind of wondering at what point you can legitimately claim a receipt is proof of ownership. Emails are very easily spoofed, printouts certainly wouldn't mean much.
I guess the combination of email + payment to GOG on the same date, but then what if you're using GOG wallet or something like that, there wouldn't be anything like it.
If the email on the email service provider is the only actual proof then that means you lose your legitimacy (proof of it) the moment the email provider's service ends.
Edit: Assuming GOG doesn't exist anymore of course.
I've just had a look at my most recent receipt and only my username appears on the receipt, so you're right that the receipt would serve little purpose in providing proof of ownership.
If GOG and my email service provider both decided to call it a day, there would be little proof of what games I'd purchased. I could still prove payments were made to GOG, but there is no way to prove which specific games each payment related to.
Makes one realise how reliant we've become on internet services, and how vulnerable we are if they ever cease.