I don't think this was the decision of anyone working at GOG. It looks the the parent company was bought out last week.
Last year, CD Projekt 'combined forces' with another Polish company called Optimus in order to go public (Optimus were already traded on the Polish exchange). From reading this article, it sounds like a LOT of senior people sold their controlling interest in the company off to a new majority owner. Someone definitely bought up a lot of shares at the end of the week, with the company's share price shooting through the roof as a result.
With the swiftness with which the site was yanked, it seems likely that some sort of restructuring might be about to happen as a result of a takeover. Apparently, Optimus/CDP had already announced some sort of a stockholders meeting and an announcement to follow this coming Wednesday. The GOG team tweeted earlier today that they will make an announcement some time tomorrow regarding what's happening.
Anyway, if it is a takeover, the actual GOG staff likely had no advance warning of what was coming. If it's a marketing stunt, it's pretty much the douche move of the year.
Meditation on inevitable death should be performed daily.