The WTO is basically a club and the rulebook is GATT (General Agreement on Tariffs and Trade). Members of the WTO are obliged to treat every other member the same. This is called the Most Favoured Nation Rule. Article I GATT.
wto.org/english/docs_e…
But there are exceptions. Mainly:
1. Customs Unions
2. FTAs
3. Developing countries.
1 & 2 are part of GATT Article XXIV.
An FTA is an agreement to remove tariff and non-tariff barriers (up to a point) but no harmonisation of MFN tariffs to 3rd countries.
A CU is similar but members operate as a bloc with a unified external MFN tariff.
FTAs didn't really get going until the 1990s before then the focus was on multilateral reductions at the WTO level.
The EU has more FTAs than any other country or bloc.
The EU's version of GSP includes total tariff elimination for countries on the UN's Least Developed Country list.
gspevaluation.com/wp-content/upl…
"..duties on goods originating in Colombia .. in the tariff lines in staging category "0" .. shall be eliminated entirely [and] free of duty ... on the date this Agreement enters into force"
assets.publishing.service.gov.uk/government/upl…