You will need to go to this location.

Hold square button to inspect loot box.

And get one finger near the options button.

After that, hold square and immediately press options.

STORY->SAVE GAME->Empty Slot->Yes.

You have to press O button and select load game.

Press X button on the slot with the latest time.

And then press X button again to confirm.

After that, go back to the glitched gold loot box.

Get hit by animals for good luck.

You have to pick up 30 gold bars.

Go to a fence (progress story to unlock).

And sell (right directional button).

Sell your gold bars for $15,000.

After that, go back to the glitched gold loot box.

Optional (save again in case you screw up.

Go to the loot box and do the save trick again.

Press options before gold is picked up.

Always choose a new save slot.

Immediately load back save (latest time).

Loot box now glitched, go back to it.

You have to take 30 gold again!