To obtain the "Gonfalon Bay's Master Burglar" achievement you must successfully steal from the luxurious safebox inside the Gonfalon Palace. First you must enter the Gonfalon Palace, which is location in the city of Gonfalon Bay in High Isle.
Gonfalon Palace location
Once you have entered the palace walk straigh ahead. There should be a door leading to the Gonfalon Palace Tower. This area is restricted, and being seen in there will give you a bounty. Once you go through the door, take the stairs to the lower level. Once you reach the end of the stairs, look to your right. The safebox should be sitting there against the wall. Lockpick and then loot it to complete the achievement.
Gonfalon Palace Luxurious Safebox
