For this achievement, you’ll have to complete all of Gaul’s side stories, which are unique secondary missions that advance the sub-plot with Gaul and help you get to know him a little better!
To do this, you must complete all 4 of his side stories. Side stories are marked by a yellow indicator above the character’s head. When a new one is available, approach them and select Side Story to begin the secondary mission.
Do note, you cannot progress the main story arc while completing a side story, so you must complete it before moving on with the main game.
Each character (Thea, Gaul, Odgen, Kora, and Eleanor) has 4 side stories to complete, for a total of 20 that all progress the Shining Kinship Stone achievement:
Like many of these achievements, you can check your progress in your Ranger Records by pausing the game, navigating to “Ranger Files” (one tab to the right), and then selecting the Ranger Records dossier (the red one on the right).
Gaul’s Side Story medal is the third silver/blue medal along on the first row, and within it, you can see how many Side Story chapters you have completed. Once you’ve completed them all, you’ll have your achievement!