Subj.
I have several games now on my comp, and wonder if I'll need to reinstall them with new client, or it's will just automatically configures and recognizes them for future auto-updates.
It also stops me from installing new games for now, as I don't want to download them twice or make some savegames-moving-voodoo-magic.
Any ETA BTW?

I see no reason why you'd have to reinstall them.

I don't think GOG Galaxy will see what games you have installed unless it looks for them in the default installation folder like "C:\GOG Games". It would probably be very unpopular if they store information on your computer such as the registry to keep track of the games as it could be considered as a DRM.

There are rumours for release or maybe an news update in August which should be any time now, if that's true.

They already use the registry, I have a lot of entries under
"HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\GOG.com"
I haven't heard anyone complain about it.

It would make sense to me if the Galaxy client simply asks the user which directory to use (and ideally if it can look in multiple directories).

I have all of my GOG games backed up on an external hard drive, and I have the ones I'm playing installed to the C: drive. The Galaxy client would be convenient if it could see both, but if not then that's fine. I'll just manually transfer and install games to the C: drive as I want to play them.

Indeed, and said entries are necessary for GOG's launcher to work at all, which is used for most - if not all - GOG games on Windows (it's run as "goglauncher <gamename>" upon which it looks up the registry key for said game under the key you mentioned, or HKLM\SOFTWARE\GOG.com on 32-bit systems).