I'm playing shadow warrior 2 and already unlocked some achievements, but I didn't get any sort of pop-up inside the game telling me that I got them.

How do I fix this?

That was it! I had disabled rivatuner before, but that alone didn't work. I had to completely close MSI Afterburner to fix the problem. Now the achievements are showing in-game.

Thanks.

Acheivements also require a persistent connection to GOG servers, and if it misses the trigger it DOES NOT recheck for it. This platform is not stable for Acheivement Hunters at this time.

(On xbox Acheivements work offline and keep a tally of progress)

I'm playing Wolfenstein: The New Order & it seems to be the same issue but add to that that the overlay option is greyed out, It did work for the other games but I don't know what's going on here anyways thanks in advance,

As you can see from the store page, the game doesn't have overlay support (it's missing from the list of game features). It also looks like the overlay won't work even when force-enabled. You can keep an eye on this thread in case a solution is posted there.

I have had quite a few GOG games registering all the previously unlocked achievements just fine after I have gotten around to remember to allow them to access the internet, so one could easily argue that the problem is not with the Galaxy API, but with how some developers have implemented their achievement tracking to entirely rely on constant internet connection.

I'm having problems with achievements now, because they don't show up in the game and there's no sound of getting the achievement when I get it... But when I log out of the game, the achievements are earned...

I am having the same issue. But only if I want to stream my games with OBS. As soon as I launch Streamlabs/OBS, the notifications about achievements stop popping out.

Yesterday I checked the problems with displaying achievements in the game, using Steelrising and Risen as examples. In Steelrising, the overlay option is grayed out, even though the game page says it supports overlays. In Risen, the overlay option is enabled and achievements are displayed in the game as they should be. So now I at least know why some games don't show achievements during the game. But what should I do to fix it? Here I have not found a solution to the problem...

Apparently it works if you force-enable the overlay.

Thanks for the tips :) I'll try it and write back if it works for me.

I don't know if I did something wrong or if the overlay just doesn't work for me in Steelrising. I did everything exactly as written in the force-enable the overlay instruction. But nothing has changed and the game doesn't show achievements when I get them...

I can try asking for help in these threads:
https://www.gog.com/forum/general/galaxy_overlay_fixes_for_games_that_are_officially_supported/post95
https://www.gog.com/forum/general_beta_gog_galaxy_2.0/galaxy_overlay_tests_and_reports_for_unsupported_titles/post1

I just installed the game again and still can confirm, that it's working fine when the overlay is force enabled. (Luckily some achievements popped retroactively the moment my save was loaded. xD)
Make sure your config.json looks like in the attached screenshot and don't forget to restart Galaxy after editing the file.