What is the famous principle of the general agreement on tariffs and trade (gatt), which means that each member nation is to be treated equally by all contracting nations?

Most-favored-nation status      

Explanation:

              General Agreement on Tariffs and Trade is most popularly known as GATT. GATT is an international organization with 160 countries as its member nations. The main purpose of GATT is to promote international trade by lowering and eliminating the trade barriers across the borders. It helps to eliminate barriers such as tariffs.

            The "Most-favored-nation status" is the famous principle of the GATT members nations. It means that each member nation of GATT has to be treated with equal by the contracting nations.

            Countries having the Most favored nation status  enjoys best trade terms which is given by its trading partner countries.