Many of the world’s nations are gathering starting Monday in Spain for a high-level conference to tackle the growing gap between rich and poor nations and try to drum up trillions of dollars needed to close it. The United States, previously a major contributor, pulled its participation, so finding funding will be tough. The four-day Financing for Development meeting in the southern city of Seville is taking place as many countries face escalating debt burdens, declining investments, decreasing international aid and increasing trade barriers.