I just logged on to gog.com and looking at my profile see my percentage complete in games I've purchased.

Seems accurate, lost interest immediately in most of them so they show 0%.

So what I buy at GOG.COM is tracking my usage?

If you don't use the Galaxy Client and instead use offline installers, then GOG is not tracking your gameplay related data.

If you do use Galaxy Client when playing games then by default Galaxy will be tracking your achievements and the gameplay time for the games you play (not all games have achievements though). However, you can disable these "features" in the Galaxy Client's Settings.

The percentage you mention is most probably the percentage of achievements unlocked in a game. I am not aware of GOG tracking any other form of percentage.

Hope this helps.

You'd think this would be obvious.

The poster started yet another thread where a half dozen already exist.

*shrug*

I thought they were tracking purchases regardless of whether you use Galaxy otherwise, how would they be able to recommend games on the last sale?

They would have to have some sort of data that they have to draw from to recommend games to you.