The gold key is dropped in a white sack by the Goblin Warlord in Vela, located on the island to the far North of the map, the one with a single building with no door and the dock with two boats. He starts out inside that building but might have ended up else where so firstly I'd go check around that area for either him or a white sack that you missed opening.
If you can't find it and think you might have gotten it and since put it down elsewhere then you have two choices, either delete the files "22.map" and "22.ent" from you save game directory then reload. This will reset Vela and allow you to retrieve the key again.
Option number two is to use Xolotl's character editor
to give yourself the key, open the char file from your savegame directory, click a blank slot in your inventory, select item type as key, choose the correct graphic, and then give it the name "Gold Goblin Key", make sure to spell it right or the game won't recognise it as the right key. Save your character, reload your game and you should have the key.