Here's the setup for this news story:
A few days ago, Green Man Gaming started to advertise a new promotion for The Witcher 3: Wild Hunt. The deal was 35% off of a Witcher 3 pre-order and made it the cheapest (AFAIK) deal in town for the upcoming CD Projekt RED title.

The problem here is that GMG was never actually given any keys for the Witcher 3, at least not from CD Projekt RED. So either GMG was selling a product they didn't have yet in hopes of striking a deal with CD Projekt RED or they were pre-emptively selling keys that came from unknown sources.

Now, CD Projekt RED has asked their fans not to buy from GMG at this time.
Rafal Jaki - Business Development Manager at CD Projekt RED
"I would kindly ask our fans no to buy via GMG at this time. We had not sold them Keys and dont know the origin of them."

Green Man Gaming issued their own statements about this current situation.

Sulyok then went on to say that yes, GMG had indeed obtained keys from third parties and other retailers. He says that these retailers were approved by CD Projekt RED and that as a result of this, CD Projekt RED is actually earning revenue from the sales of these games as the additional discount is absorbed by GMG.

CD Projekt RED said that they actually receive "zero" revenues from those types of sales.
"We would heartily welcome a renewed dialogue with CDPR, and are keen to continue to not only support the launch of The Witcher 3, but to keep celebrating and bringing the whole catalogue of CDPR titles to a worldwide audience, as we have done since 2011," Sulyok said.

As it stands right now, your safest bets for pre-ordering The Witcher 3 on PC are GOG, Steam, Origin, or any of the other retailers listed at the pre-order page on the official Witcher 3 website.