Myanmar entered the United Nations and the GATT in the same year, and the WTO on the day it opened
Background
The United Nations list of Member States gives the entry Myanmar Date of Admission: 19-04-1948. On trade, the World Trade Organization's member information page states that Myanmar has been a WTO member since 1 January 1995 and a member of GATT since 29 July 1948. Because 1 January 1995 is the day the WTO itself came into being, Myanmar is an original member of the organisation rather than a later accession, having carried its membership across from the General Agreement on Tariffs and Trade.
In plain words
GATT General Agreement on Tariffs and Trade — The post-war trade agreement that governed world trade rules until the WTO replaced it.