Gold Medal Checklist:
- Peacefully convince the Chelonians to let you take Jamie
- Return Jamie to Mary within 2 minutes
At some point during Chapter 2, Arthur will receive a mysterious letter at his tent. Read it to begin the mission, his love from a distant past has written to him and wishes to see him. You'll need to make your way over to Mary's house situated to the north of Valentine and knock one the door. It appears she wants to enlist Arthur's help in tracking down her brother who has gone missing.
Your next destination lies to the east of Cumberland Forest, on top of a cliff. Leave the horse and approach the group of Chelonians by the cliff's edge to find Jamie. When given the choice to answer the Chelonian Master's question, select "Undecided" followed by "Purpose" to end the discussion peacefully. Jamie however has other ideas and takes off on his horse, quickly mount yours and give chase. You only need to keep pace with Jamie, you can't actually catch him during this chase and eventually you'll come to a standstill after crossing the railway tracks. You'll be introduced to the dueling mechanic here, slowing press the [R2]/[RT] button to fill the circle in the bottom right, this will give you more time in Dead Eye to shoot the gun out of Jamie's hand.
Having come to his senses, Jamie will agree to return to Mary with you at the station and you'll need to make the journey within 2 minutes if you want the Gold Medal. Seeing as Mary will be at the station, ignore the marker and follow the railway track, this will save time provided you Gallop the entire distance. When you finally arrive, head inside to meet with Mary as brother and sister depart on the next train, leaving poor Arthur confused in love once more.