The world organisation that replaced GATT is:

The General Agreement on Tariffs and Trade (GATT) was an international trade framework established in 1947 to reduce tariffs and other trade barriers among member nations. It operated through a series of multilateral negotiating rounds over nearly five decades.

The final and most ambitious of these was the Uruguay Round (1986-1994), which expanded the scope of trade negotiations beyond goods to include services, intellectual property, and dispute resolution. The Uruguay Round concluded with the Marrakesh Agreement in April 1994, which established the World Trade Organization (WTO) as a permanent international body to replace GATT.

The WTO officially came into existence on 1 January 1995. Unlike GATT, which was merely an agreement without a formal institutional structure, the WTO is a full international organisation with its own secretariat, a binding dispute settlement mechanism, and a broader mandate covering trade in goods (inheriting the GATT framework), trade in services (GATS), and trade-related aspects of intellectual property (TRIPS).

OPEC (Organization of the Petroleum Exporting Countries) deals with oil production coordination, not general trade. UNCTAD (United Nations Conference on Trade and Development) focuses on development-oriented trade issues but did not replace GATT. The IMF (International Monetary Fund) addresses monetary policy and financial stability, not trade regulation.