Kismet: If Galaxy doesn't allow the download of the stand-alone installers, updates and extras while automatically checking their integrity and re-downloading only the problematic chunks in case of download errors then no, it's not really the same thing.
The way it looks, Galaxy will work like Steam and install the games, leaving downloading directly from the website as the only option for those wanting to get the stand-alone installers and updates, which is not as good an option as the current GOG Downloader for the features mentioned above.
It's been asked a few times already if this will be the case, the fact that it's a question that's been either ignored or side-stepped is reason not to be particularly optimist.
To me such a change would make a significant difference, and would likely make me prefer the Humble Store, at very least because they provide MD5 checksums to verify downloads (and Linux binaries, often not just for Debian distros).
And of course, Steam keys for achievements and playing multiplayer games with Steam friends, should I start caring about either feature.
EDIT: Missed HypersomniacLive post as I was busy writing mine, sorry.
No need to apologise, these are exactly the worries I have for the reasons you listed.
I'm also worried that support for the traditional standalone installers and patches will fall behind over time, and everyone will end up having to use the GOG Galaxy client if they want to play an up to date game, even if they don't endorse the way it works.
Of course we have almost no info to base our worries on, I just think that if the functionality of the current GOG Downloader has been incorporated to the GOG Galaxy client at the design level, our question is a very simple one to answer.