So I'm finally able to install Prey after several closes-and-reopens of Galaxy. I decided to do a little testing, and to my surprise, the save files from my Steam playthrough 4 years ago are still there. I loaded a save, which was very near the end, and got no achievements. I also took a look at my profile, usually there's a milestone that says "(username) started playing (game name)" along with the game's banner. There isn't. But the game still appears on my recently played list, with gameplay time recorded and overlay working in-game.

I know GOG had a major outage recently, which is why I'm asking if this is outage-related and the connection between the apps are not fully restored yet. Or does Prey have a "unique" code in the game that doesn't allow you to earn achievements (and in this case, a GOG profile milestone) if you reused a save from long ago?

There's no achievement for simply finishing the game; you must meet certain conditions (e.g. finish the game without acquiring any Typhon power or human ability to unlock No Needles). Your profile page currently shows 10 minutes spent playing Prey. "Started playing [game title]" is added to your profile when you reach 15 minutes.

The save that I loaded and finished met several conditions. On it, back then I unlocked the achievement where you use only Typhon powers and killed all humans. And I made sure there were no human powers unlocked, on my powers menu and stat screen.

It always has been spotty, since the very day Galaxy was rolled out for public testing.

GOG has legions of games that have legions of bugged Achievements in them.

"Galaxy's connection with GOG" is a totally separate issue from GOG's countless number of bugged Achievements, although the thread title and original post seem to be conflating those two things together, even though they cannot really be conflated.

Not every game will pop a ton of achievements when you load a save from Steam. I've had some do it, and others have not. It really depends. So I wouldn't assume there's a Galaxy problem there just yet.

Some games do have broken achievements on GOG like Wolfenstein The New Order, but Prey isn't one of them.

I just tried again last night and it seems yeah, there are several different triggers for the achievement unlocking instead of simply the game end screen. I got the no needles and escape with Dahl achievements after loading an older (but still near the end) save. I assume those achievements where you kill all humans for example only shows up a short while after killing the last one.

There's nothing wrong with the achievements for Prey Digital Deluxe Edition.