This quick Fable 3 Gold Key and Gold Door location guide should help you find them all. There are only five Gold Keys and four Gold Doors (plus one Gold Chest), and unlocking some of the Gold Doors with a Gold Key will even grant you a coveted Legendary weapon.
Gold Key 1: Driftwood
You will first need to go to Millfields and complete the 'Restoration' side quest by paying 750 gold to repair the bridge to Driftwood. Once in Driftwood, locate and complete three more side quests (An Island Getaway, Pest Control, Giftwood for Driftwood) that will grant you access to the coastal islands. On the island you'll find a flit switch which you can follow and shoot the switch as it leads you up the mountain. At the top you'll warp to the Gold Key.
Gold Key 2: Mistpeak Valley
Chillbreath Cavern is your goal, located on the side of the map away from the Demon Door (two other caves are near the door). Once in the cavern, follow the main path uphill until you find an exit and follow it out. The same general direction will lead you down a hill where the Gold Key awaits in the ruins.
Gold Key 3: Aurora - The Veiled Path
Find the Enigma area of The Veiled Path and shoot the flit switch to get into the next room. Use the Fireball spell to light the torches you see to proceed. In the next area with colored floor panels and flames on the wall. The flames reveal the pattern to step on the panels: Yellow, Blue, Yellow, Red, Blue, Red, Blue, Red. Once completed, the next door will reveal the Gold Key.
Gold Key 4: Aurora - Shifting Sands
Complete the 'A Key To A Greater Key' in Aurora's city. Once complete, the guy who gave you the quest will sell you a key for 4,000. After buying the key, head to Shifting Sands and run across all the sand dunes. On the other side, hang a left at the cliff face and locate a path near the ruins. You'll find a door that requires a key, the one you just bought. Fight off some enemies on platforms to reach the Gold Key.
Gold Key 5: Sanctuary
As the Ruler of Albion you'll have access to the treasure room in the Sactuary (i.e. pause menu). Once in you'll notice a Gold Key up high that will require $5 million in gold mounded up to ascend and reach. Once you have the key, move all the money to the Albion Treasury in the castle and a chest will appear that the Gold Key unlocks. Inside is a Legendary weapon.
Gold Door 1: Mourningwood - Ossuary
Gain access to the Ossuary through a graveyard door after completing the 'Bored To Death' and 'Gone But Not Forgotten' quests. Hang a left after entering to spot the Gold Key door that hides a Legendary weapon behind it.
Gold Door 2: Brightwall Village - Reliquary
Complete the 'Special Delivery' (Dweller Camp) and 'The Ancient Key' (Brightwall Academy)' quests, the latter quest showing you a secret library in the Reliquary where a flit switch sits on a wall. Fire four shots at the switch with any ranged weapon to activate. Head up the stairs, hang a left through the door and melee attack the next flit switch twice; once while high and then again after it drops. Beat the hollow men and a path will form to the Gold Key door which houses five chests containing pieces of the Highway Man outfit.
Gold Door 3: Bowerstone Castle - Catacombs
Head to the Catacombs under the castle and the door is obvious. It houses 30 Guild Seals, perfect for your next trip to the Road to Rule.
Gold Door 4: Sunset House
A relatively easy door to find. Enter through the main gate and head right towards the back area. The Gold Key Door is across from the gazebo and behind it is another Legendary weapon.