First thing to note with this achievement is that the description is slightly misleading. You don't actually need to do any Gold Hoarders voyages to earn this achievement. The commendation in question is simply for selling loot items to the Gold Hoarders. They accept a variety of items including chests and treasure artifacts.
Every time you sell any of these items the gold is added to your tally that can be found by pressing
, picking the gold hoarders (left icon) and looking at the second item on the top line. When this value reaches 150,000 you will earn the commendation and you can now buy the title "Keeper of the Glittering Hoard" from the clothes shop.
The basic method of obtaining chests is to do the gold hoarder missions. These come in two basic varieties, X marks the spot maps which can have multiple chests on one island and riddle quests which only ever give up one chest. Many people cancel voyages with only riddle chests and don't bother visiting islands with only one x marks the spot. But there is an achievement for completing 500 voyages so don't be too short sighted rushing for 150,000g; play the long game and aim for 500 voyages.
Other chests can be found as random loot on islands. Keep your eyes peeled as you never know what you might find.
Treasure artifacts usually appear as shiny objects (like a message in a bottle) washed up on beaches.
Ship wrecks often contain a couple of ship wreck chests and you need 1000 of those for the "Hoarder of Barnacled Gold" achievement so it's always worth going for a dive when you see the large groups of birds.
And finally skull forts. Complete a fort, grab the treasure and sell the skull chest for a major boost to your gold hoarder earnings. Skull forts are PvP hotspots so make sure you know what you're doing and take a good crew.
Note. I unlocked the commendation but the achievement didn't pop until the next day. I had probably sold about 155,500g of loot to the Gold Hoarders by this time. If it doesn't unlock for you when you complete the commendation don't worry, keep turning in chests and it should pop soon.