The only way galaxy knows you are playing a game is if it's running in the background, if it's not running in the background it can not track time played in a game. The games do not come with added software to track when playing. I have played several games with galaxy off and logged in to see if tracked my time, it does not.

That is understandable. But why would you need to be connected to the internet for that (which is what the OP asked)?

So if I install Galaxy after playing almost all TW3, I will have my achievements show up? Kind of neat if I'd actually care about achievements :D

I've got 2 other games where it works, and I can see my achievements on the galaxy page (by clicking on the small "trophy" symbol). Don't know about TW2.

I missed the internet part and thought he was just asking about Galaxy running. I have not tried running Galaxy without being on the internet so I don't know if it tracks time played then or not.

TW2 page says 0%, I guess indeed it works in some other games. not fair, gog :<
edit: if you go to your settings page, is the option for achievements greyed out?

No. I played close to 20 hours of Pillars before I installed Galaxy, it showed 0 hours played after I did install Galaxy. No time put in prior to Galaxy was tracked.

I'm rather curious about achievements than playtime. I wonder how many i unlocked without even knowing :)

There's no way Galaxy would know what achievements you've earned if you did not play with Galaxy running in the background. If there is a way then GOG would have had to install software into the game file that tracks what you are doing and relay that info to Galaxy when it's installed, and I certainly hope that is not something they would do.

I was hoping the game itself tracks them and Galaxy just showcases them.

I just checked, and I have 16% of the achievements for "renowned explorers". And the latest 2 are displayed under the time played and % achievements. Same for "the witcher adventure game". All of the others indicate 0% achievements, but I tend to avoid Galaxy for some other games I play now (like pillars of eternity) as its updater tends to screw up for them, so that might be the reason.

For the settings : I didn't see any option about achievements in my "settings and privacy" page. On what thumbnail is it?
It doesn't.

the setting page for galaxy client I meant. Is it greyed out?

You're talking about the setting and privacy page which can be accessed on galaxy by clicking on your profile, or is there another one?

I am pretty sure, that some games track achievements and in-game time themselves. I have started Lords of Xulima in offline mode. Even though I have installed Galaxy, I always played Xulima with Galaxy turned off.

Then once I have started the game from Galaxy and after I quit it, the time played changed from 0 to 35 hours.