I'm having the same problem right now. I try to checkout using PayPal. I enter all my login information and PayPal begins processing, then I'm sent back to the GOG checkout screen. I've done this on multiple browsers and on multiple devices, to no avail.
I've used PayPal to make all of my purchases before, so I'm very confused as to why it suddenly doesn't work. The problem is compounded by the fact that, despite my best efforts, my bank will not process transactions straight from GOG themselves, hence the reason I use PayPal in the first place.
I'm hoping to pick up FEAR and Tales of Monkey Island before the sale ends, but that seems less and less likely as I continue to attempt the purchase throughout the weekend.
EDIT: I just made a PayPal purchase elsewhere that went through without any hiccups. This leads me to believe there is something wrong on GOG's end.
Post edited February 12, 2017 by Vampire-Jekyll