JudasIscariot: Please note that there are no authorized GOG key resellers.
How come Greenman offered Gog keys? I thought they were a legitimate site, but they were selling Witcher 3 GOG keys at a 33% discount a while ago.
As for grey market keys in general, it's always a risk to buy those. Most likely you'll get a legitimate key, that has either been bought from a region with lower game prices and/or from a boxed copy. But the important thing to note here is that you can not be sure of this, those major grey market sites let other people sell games without any real method for checking if those sellers are honest, or if they've been using stolen credit cards to get the keys. For the record, yes, I've bought from the grey market in the past, and not had any problems, but in the last year the amount of incidents related to stolen or otherwise illegitimate keys seem to have gone up.