Spend a total of 100,000 Gil in the game's shops. Both Chocorina's shop as well as the currency exchanger in the casino, Serendipity, count toward this trophy. Note that unlike Final Fantasy XIII, Gil is awarded after all battles in Final Fantasy XIII-2.
You should have earned a total of 100,000 Gil before the end of the main story, if not shortly thereafter. If you want to hasten this trophy's acquisition, simply sell off all of your items and equipment to ramp up to 100,000 Gil. Spend enough Gil to make the trophy unlock, then just load a previous save (in order to recoup the Gil lost).