Lucumo: Not really. I use Galaxy because I like to know my playtime, convenience comes after that. If it was mandatory though, I certainly wouldn't be buying any games here. Just because you use something, it doesn't mean you don't heavily support the alternative.
Breja: You say so, but the fact is people using Galaxy, whatever their reasoning, are hardly sending a "anit-client" message to GOG. And people using the obligatory clients of GOGs competition are sending a very clear message that either they like or outright want to use clients, or at the very least they will put up with using them to get the games they want. That's really the message the gaming community as a whole has been sending the publishers and stores for a long time now. And really (even if its not true in your case) GOG has every reason to expect that should Galaxy become mandatory, people like you who already use the client would not really stop buying games here, at least not for long.
The way GOG responed to the profiles debacle recently makes it pretty obvious that the forum community here, which accounts for the most "old school" part of GOGs user base, is seen by GOG as an annoying, vocal minority. And as we can see even among us here clients are being embraced by a lot of people, including services where they are obligatory.
That's definitely true but like I said, they also aren't necessarily sending a "pro-client" or "client-only" message. People tend to overlook that, following their assumptions. Of course, this is not the case when it comes to other clients (Steam, Origin etc). I mean, keeping in mind that I don't use Steam and the like because I consider clients to be DRM (in addition to their other DRMs), I would definitely stop buying, forever. Pretty sure I'm not the only one using something optional who would then have objections when it becomes mandatory. Still, people like me and people who don't use anything are probably in the minority. We don't have any data, so we can't really draw any conclusions.
Something tangentially related: Using Galaxy for multiplayer is fine. I really want to know what people who whine about that have to offer as an alternative (apart from direct IP connection and LAN). Last time I asked that someone, that part got ignored.