To unlock the first quest, visit the Hall of Monuments and watch the dialogue where four Ebon Vanguard members notify Gwen about what happened to Lieutenant Thackeray and Captain Langmar after the Battle for Lion's Arch. A scout also appears in Ice Cliff Chasms, just outside the Eye of the North, who tells you to search for Keiran in Kryta.
Your first clue is found in Watchtower Coast, North-West of Beetletun — it is an Embedded Arrowhead, which starts the quest The Arrowhead. Complete it, equip Keiran's Bow given by Gwen in the Hall of Monuments, speak to the Scrying Pool and choose "Show me the story of the arrowhead." to enter a solo mini-mission, Auspicious Beginnings.
Your second clue is a Tarnished Emblem, found in Majesty's Rest, just East of the portal from the Sage Lands. An easy way to reach it is to head East from The Wilds. The Emblem gives you The Tarnished Emblem quest, which takes you back to the Hall of Monuments. Once complete, equip Keiran's Bow again, speak to the Scrying Pool, and choose "Show me the story of the tarnished emblem." to enter the second mini-mission, A Vengeance of Blades.
Your third clue is a Broken Sword, found in the center of Ettin's Back. Head South from Ventari's Refuge, go East through the passage where Hember is, and turn back North, follow clock-wise around the large ravine to find the sword. It gives you the quest The Broken Sword. Repeat the steps as before and choose "Show me the story of the broken sword." at the Scrying Pool to enter the third mini-mission, Shadows in the Jungle.
Your last clue is a Shining Blade Cloak, found in a ravine that runs North-South through the center of Watchtower Coast. It gives you the quest The Mantle's Guise. Follow the steps as before and choose "Show me the story of the Mantle's guise." to enter the last mini-mission, Rise.
After you complete the last mission, Gwen tells you she wants to meet Keiran on his way, and gives you the quest Reunion.