JudasIscariot: I doubt that there will be some sort of bloat or that any code added for Galaxy support will make the installers noticeably larger.
Telika: I don't think I'm bothered by size as much as by the principle of some gog-specific code (in the sense of "device") being imprinted on the games... Their gogification, in a way. Okay, we have gog installers with gog eula and gog ads displayed during the install, but that's still almost "outside" the game, and, well, it's basically just a visual label during the installation.
Having the games' code including some gog-system, to be compatible with a gog client (which I can't help imagining more short-lived than the game itself) does feel a bit intrusive. It's maybe more symbolic than rational, but it make me feel that the gog games are less "intemporal", less "preserved", and more "recuperated, reappropriated and incorporated into a new system".
I'm not yet sure of how much this makes sense, and how well I pinpoint my unease. And maybe I'm the only one feeling that way. But, whether I used the GOG downloader or not, I liked the idea that this gog downloader was completely "external" to the game...
Actually, game devs are often quite relieved when any multiplayer stuff is done for them. Consider it as they don't need to bother with account management, lobby and matchmaking, because someone has already done the work for them. The devs of dawngate for example, said that they are using the origin accounts, because it saves them a lot of work. Divinity Dragon Commander's online mutliplayer outside of steam is also inexsistant.
So it's not that the devs are putting extra work into it, quite the opposite, they have less work. The only question is which option they choose to save them the work, whether it's galaxy, steam, origin, etc. Galaxy, if it works as promised, is a good option, because it's compatable with the other ones as well.
Anyway, in order for this to work, Galaxy needs to be reliable and bug-free. To achieve this, I'd recommend a huge beta-test. Best would be GOG programs or licenses a small game (e.g. Sumrai Gun) to test the whole thing way in advance. If Galaxy is a buggy mess when released, devs will surely skip it.