As the subject says:
how can I disable recently introduced gfx HW acceleration in GOG Galaxy Client?

Nvidia drivers + some GPUs (I have GTX 980m) have issues with CEF (Chromium Embedded Framework) component used by GOG Galaxy, when gfx HW acceleration is enabled (corrupt rendering, colors, fonts):
https://image.ibb.co/gOWKO7/image.png

It's a generic CEF issue, now present also for GOG Galaxy since gfx HW has been enabled with last release.
Is there a way to disable it as you can do with Chrome in advanced settings?

Windows 10 x64 FCU 1709 (16299.248), Nvidia drivers 390.77 WHQL, GeForce Experience 3.7.0.81

Set galaxy to run with intel card in nvidia control panel (manage 3D settings),

I don't have an IGP and don't use Optimus.

Have you tried what was suggested here?

Thank you. FXAA is off, and as you cans see from pics it's not only a blurry text issue but a real rendering issue (colors are incorrect, purple/green pixels everywhere...). Already tried also to reinstall video drivers from scratch after DDU using default Nvidia settings, but nothing helped and the issue is reproducible only with some web-based tools using CEF (Chrome Embedded Framework) when their gfx HW acceleration is enabled.
I suppose many can't reproduce this issue because:
a) most people use Optimus mode (automatic IGP <-> GPU switch between power saving desktop mode and 3D gaming)
b) they have an old video driver or gfx card (?)
c) they have an AMD gfx card
d) this issue might be GPU + drivers specific (Nvidia should take care)

NEWS:
forcing Anisotropic Filter to DISABLED in the Nvidia panel for "galaxyclient helper.exe" solves the corrupt GUI rendering with GOG Galaxy client (that recently introduced CEF based gfx HW acceleration)!

This workaround seems to work also for other CEF-based apps including Google Chrome browser when gfx HW acceleration is ON!

I'm perplexed at the idea that a client is using anything requiring hardware acceleration. A launcher with cutting edge graphics? Please, someone, show me a screenshot of this bloat so i don't have to install it myself to see it run at 3fps on my screen.

CEF is not an optional 3rd party resource. Is a framework (included in the app distribution and compiled with it) used by many web-based apps today, able to use HW gfx acceleration when available.
Some of these apps have an option to enable/disable acceleration (e.g. Chrome) but many don't. In this case, if video drivers and HW acceleration don't work so well together there can be some visual issues...

CEF: https://en.wikipedia.org/wiki/Chromium_Embedded_Framework

Why anisotropic filter was enabled for Galaxy client?... (not on my pc)

default is disabled, not enabled

I see you found a workaround in the mean time, and the attention of an nVidia representative; hope they fix the issue soon.

Default is "Application-controlled", not disabled which changes also other 2 settings below... and many users use to force AFx8 or x16 (it's often better than in-game settings and gives more detailed textures).

Very, very bloated. The grammar of your reply implies that it is somehow a counter-argument to my implied-argument, when in fact you're really just confirming my suspicion: galaxy has a large list of unnecessary code, which causes it problems, simply because they're using the wrong tool for the job. Hardware graphics acceleration rarely is even used for 2d graphics. Sure, the drivers get called ,but a good junk of it ends up handled by the drivers, not the GPU (this is why my computer can handle skyrim, but not Terraria, Starbound, and other 2d games that have lots of sprites).

EDIT: To clarify, this unloading doesn't really reduce or prevent the issues from the mismatching, though. Graphics drivers aren't meant to handle multiple parts (contexts) of the screen while the rest of the screen is pre-rendered before the drivers get their hands on it. I'm kind of surprised it gets glitchy instead of just having really poor performance, though. Could be the type of stuff chrome's doing itself before passing it off to the drivers. If it's trying to overlay graphics accelerated ontop of pre-rendered without actually separating the regions with a context, this would make perfect sense. Thanks to "layered windows" i did pull it off back on windows XP once and had 3d graphics rendered (via OpenGL) on the desktop and use keying to show the rotating borg cube without the rest of the window, so you could see the words on the web pages around the cube itself. Incredibly slow, though, but that was on a pentium 4.