Curse of Opulence's triggered ability resolves after attackers have been declared. Any costs to attack must have already been paid before the attacking player creates the Gold.

A player is attacking another player if they control a creature that's attacking that player. If no creatures are attacking the enchanted player as a Curse's ability resolves (most likely because they've left the battlefield), only the Curse's controller performs its actions.

Each Curse's ability triggers only once, no matter how many creatures are attacking the enchanted player.

The triggered ability of these Curses won't trigger if only a planeswalker controlled by the enchanted player is attacked.

If you enchant yourself with one of these Curses, you'll get its effects whenever another player attacks you.