Gordon the prince
Default weapon: spear and shield
Kashuan Keep
Bring the sad news to Princess Hilda and she will tell you that you need to find Cid immediately. Head straight to Kashuan Keep and you will see Princess Hilda and Cid upon entering.
Use the Goddess Bell on the locked door at the top, above Sunfire, and the door will unlock itself. You will find Prince Gordon upon entering and he will join you to find Egil's Torch. Get all the treasures in this map and finally the Egil's Torch. After retrieving the Egil's Torch, exit to 1F and use the torch on the Sunfire. The Sunfire will be transferred on Egil's Torch. Exit the Keep and you will see Cid's airship get captured by Dreadnought.
Dreadnought
Head back to the Altair and speak to everyone in the room. You will find out that Princess Hilda and Cid have been captured by the Dreadnought, but it requires replenishment and the replenishment location is to the Northwest of Altair. Head Northwest and enter Dreadnought. Go through the map to find Cid and Princess Hilda. Cid will tell you to throw the Sunfire into the engine. Go to the 5F which is the engine room to throw the Sunfire into the engine. A Dark Knight will appear and ask what have you done to the engine. Maria will recognize the voice but there is no time to talk here as the ship is about to blow. Your party will auto exit the Dreadnought and head back to Altair.
Go into the King's room and you will find that the King is about to die. Before you do so, unequip Gordon's items as he will leave you after this section. The King will ask Gordon to take lead of his army. He asks of Minwu to break the seal binding the Ultima Tome. He asks Firion to head to Deist, home of the Dragoons, to search for last standing Dragoons and their Wyverns. Lastly, he asks that you all take care of Princess Hilda. After which, he leaves the world. Gordon and Minwu leaves the room to handle their tasks and you are back to 3 party members again.