Twenty-three governments signed GATT at Geneva in 1947
The General Agreement on Tariffs and Trade was signed at Geneva on 30 October 1947 by 23 governments, which the WTO counts as GATT's founding members. It came into force on 1 January 1948. GATT ran world trade rules until the WTO took over in 1995. The answers here use the names the governments signed under. Three of those names have since changed while the WTO treats the country as the same party, and those later names are accepted. One signatory dissolved in 1992, and three withdrew within four years of signing.
Name them all first. Then try the harder part: name the European, American and other signatories that stayed in GATT, the four whose 1947 names have gone, and the three that withdrew.