Hello,
I have one question regarding galaxy overlay and the achievements popup messages.
So why doesn't the Galaxy overlay work when I have Riva tuner running ? Shift + tab does nothing and also the achievement messages don't appears in game. But there is a workaround: if I don't have Riva tuner running and I launch a game everything is working fine and if I alt+tab and run Riva tuner than the galaxy overlay still works and riva tuner overlay works fine.
SO they can work fine together but for some reason I can't have riva tuner running before I start a game...why ?

Hello,

RivaTuner has its own overlay. Apparently, if it hooks into the game first, GOG Galaxy's overlay no longer works.

You can try reporting the issue to the developers of RivaTuner. When I was having problems with GOG Galaxy, RivaTuner / MSI Afterburner were mentioned by support as programs that can prevent the overlay from working. So I don't think GOG have any interest in altering their software to work with these utilities (if that's even possible).

they both use the same port

why should gog pay staff to recode Galaxy... there are lots of other busted things need fixing before we care about RivaTuner imo

You can enable a fps counter on the GOG overlay (CTRL+SHIFT+TAB if I´m not mistaken).

Yeah that works if you only want to see the fps counter....but rivatuner is much more complex...it shows RAM/VRAM usage, frequency, voltages, temps, frametimes etc. It's very useful when troubleshooting problems in a game or when doing some basic overclocking.
Idk why it works fine with steam overlay, origin and ubisoft but it causes problems with GOG. Anyway don't care that much as long as I can make it work if I start it after the game loads. :)

You can also use the Xbox Game Bar as an alternative. It works in most games.

I think I managed to get it working!
(GOG Galaxy 2.0.53 beta; RTSS v7.3.3)

The first thing I tried was forcing overlay using the steps in GOG developer documentation - paragraph Testing Overlay Locally:
Open C:\ProgramData\GOG.com\Galaxy\config.json and put "forceEnableOverlay" : true, e.g. on the second line. Don't forget the comma on the end.

Restarted GOG, overlay didn't work. But I noticed that while it doesn't show up, after pressing Shift+Tab, the game cursor changes to normal Windows cursor, so it actually tries to do something but fails)

Then I found one interesting thread on RTSS forum.

NOTE: There are loads of other threads on editing Global profile of RTSS and adding bunch of DLLs to InjectionDelayTriggers variable. I tried that and it never worked, but doing this might be necessary step too. I don't have original file, so I can't try it with original value. My current state of this variable is:
InjectionDelayTriggers = IGO64.dll,overlay64.dll,GameOverlayRenderer64.dll,Galaxy.dll,Galaxy64.dll,EOSOVH-Win32-Shipping.dll,EOSOVH-Win 64-Shipping.dll

But the thread I found mentions different variable in the Global file - UnconditionalInjectionDelay.
So I opened notepad as administrator, opened C:\Program Files (x86)\RivaTuner Statistics Server\ProfileTemplates\Global file and changed UnconditionalInjectionDelay=0 to UnconditionalInjectionDelay=15000 - the number means 15 seconds delay before RTSS tries to inject its overlay into a game, I also found other thread with 10000 as value but 15000 works well for me.

I restarted MSI Afterburner+RTSS, restarted GOG Galaxy. And after launching a game, GOG overlay pop up with keyboard shortcut showed up! Shift+Tab works as well and my keyboard shortcut for RTSS overlay works too!

I have currently installed only two "pure" GOG games. It works perfectly in Hollow Knight. In Grim Fandango, GOG overlay works too, but RTSS overlay is distorted - but I don't know whether it is because of this "hack" or it was like that before.

I'd love link to my sources too but I'm unable to post links.

Rivatuner messes with a few old games. Sacred 2 comes to mind. It won't launch with it running. I love it though.