Galaxy already "installs software into the game file", as in, the game has to implement achievement hooks. And the game can be made to track achievements internally, then submit them to Galaxy. It's not how it's currently done, though -- but it's not impossible.

Galaxy doesn't track game time but it most certainly does update the unlocked achievements despite not have it running while playing the game. Atleast thats the case with The Witcher 3 as I haven't tested any other games but I never kept Galaxy running while playing the game and it shows 0m with 28% achievements unlocked, screenshot attached.

OK, I just checked : The only feature that is not greyed out is the one that is not implemented yet (overlay). All those that are active (achievements, game time tracking, auto updating...) are greyed out and thus cannot be ticked on nor off (on my PC, they are all greyed and ticked off, but still active)

Weird

I can say I'm not a fan of Galaxy installing extra files into the games to track achievements. That should only be done if running Galaxy, otherwise I think it's intrusive. That's just me though.

I agree with you and was actually quite surprised to see the achievements being unlocked without even running Galaxy while playing. Seems like just having Galaxy installed will unlock achievements even if some just want to use it as an installer or downloader.

games all track achievements themselves because how else would a game know when you've done something to trigger the achievement?

IE if game's got a Shoot 10 Tomatoes! achievement, how the hell would Steam know what's a tomato or what isn't? Or that you've even shot one? So games have to track achievements on their own but they update Steam. With GOG, I guess GOG decided nobody would care about achievements not being tracked (a fair assumption). Also, they know that their games exist in a client-optional ecosystem where it is not inconceivable that someone who would like achievements ends up playing without the client or changes their mind about achievements and so on, and so decided to implement in a way that stores the data for later client updating. I don't know how Steam game time works but it could be that game time, as it is a much easier thing to account (Galaxy just needs to track how long the binary it spawned or createprocess is running) it just does it at runtime.