In some cases at least, the decision whether to use the definite articule was made through consultation between embassies of Anglophone countries and officials of the country concerned. This was done after Zaire (originally the Belgian Congo, then the Congo) took back its former name after the ouster of Mobutu, who'd chosen to rename it Zaire. The decision was made at that point to call it "Congo", not "The Congo." I have the impression the same procedure occurred in Gambia and Sudan.