I ordered a couple games a few days ago, and a 404 error showed after clicking through paypal. Now the amount has been deducted from my account, but the games don't show up in my library.

I've sent an email to the "contact" section, but haven't gotten any reply. What should I do?

You could contact someone of the staff directly. However if you haven't seen anything in your email or PM box, it might be that the support ticket didn't go through. I'd give it an extra day and then ask someone if your ticket is on the registry.

Did you get a receipt email from GOG?

For the question about the error, yes I got a form response saying a real response would come asap, for the order no I don't think so. I got a receipt email from Paypal, though.