Is the "Odyssey of Anton" achievement bugged in the GoG tracker? I followed a guide to find Anton every time it was possible and if I understand correctly, the achievement should pop up when you either finish his potion quest in Rolent or when you talk to him while he's crying in the bathtub afterwards. I talked to him even in Grancel in Chapter 8 and I still got nothing.

However, the achievement must be unlockable somehow, since about 13% of players have it on GOG.

Never mind, turns out I just can't read properly. I misread the guide I was following. I can confirm that the achievement is not bugged and you can get it on GoG too without any issues. You don't even need to track down Anton every time. Just follow the "minimal spoilers" guide for Trails SC (check "100% achievement minimal walkthrough", can't post the link for some reason). I only talked to Anton at the times specified by this guide, and the achievement unlocked.

Also, you don't have to worry if you missed talking to him at some point, as long as you have saves from that chapter. The "talked to Anton" counter is across the entire game. So if you missed him in chapter 2 (like I did), you can easily reload from chapter 8 to chapter 2, talk to him there, and the missing conversation will trigger the achievement.