No, but it comes with one doesn't it? Witcher 1 & 2 you can redeem your Steam version on GOG.

http://www.pcgamer.com/gog-release-witcher-2-sales-stats-steam-dominates-all-competitors-combined/

Steam's huge userbase is not a joke. Witcher's publishers may claim all they want now, but without Steam sales they're nothing.

(I'm only talking about PC sales, consoles are different things altogether)

It's from 4 years ago. Since then the situation could have changed. Steam surely is still dominant but maybe not that dominant as it was 4 years ago. I guess there is no newer source including Witcher 3?

Steam would have 125 millions active accounts ( understanding that some users may have quite a few) , of which 5 to 9 million would be active at any time. Gog does not communicate on its user base, but is generally considered to have grown faster than the market leader. Now, one will note that these user bases are not mutually exclusive and that what matters is the core of the user base , those active enough to buy regularly.

Is it though? Steam's concurrent user base has grown from 6 million end of 2012 to 8 million mid 2014 to 10 million concurrent users a month or so ago.

Granted, it's easier for GOG to multiply it's userbase, but apart from the spurt Witcher 3 has perhaps just given them is there any real indication GOG has been growing at a higher pace? (Not denying it's growing, clearly it is)
I'd really be interested in seeing those numbers but I doubt we'll get more than a general statement like 'We're excited to see xxxxxxx users are playing Witcher 3 through GOG/Galaxy!'. Particularly with how the only place to get Witcher 3 on Steam was to buy it on Steam I would imagine the numbers should be quite favorable for GOG, but then again, even if 60% of the revenue is from non-Steam sources, if 40% still is, is that really something you want to show to the public, given they're trying to compete.

With Galaxy being a beta client and optional, a measure of its concurrent user base is not really interesting at this stage. And before it, there was not such possible measure, the closest being concurrent logins on the website.
On the other hand for Steam it's very relevant. But an active account is still not a productive account... So, what would be interesting is to have the number of purchases for GOG and Steam, and that we don't have anyway...

Simple .Ask GOG support :D

Or rather CDP support. But I'm too lazy. I don't want to buy Witcher 3 right now, I still wanted to wait a bit. Maybe someone else does. Or GOG.com makes a sale.